Nope, this question hasn't been asked before!
Though the answer is most likely disappointing:
There's no canon explanation! ^^'
Me and Kai never got around to settling on one.
Originally it was that Kai discovered cute and romantic sketches of himself in Vivi's sketchbook and decided to try a move with this newfound knowledge that she seemed to have feelings for him.
But we ended up wanting to scrap and retcon that to something else where the two had a heartfelt moment after Vivi's previous relationship didn't work out and Kai was there to support her through it. But never did that either.
So right now, there's no set in stone reason as to why they got together.

🌺 I can~
I don't know how you'd differentiate how good someone is at it sooo...I dunno? I'd say I'm decent! I still struggle, gag and choke, but that's part of the fun of doing it, is it not? It certainly is for me~
As for how long I can hold a deepthroat uh...I never really timed, and it would depend on the circumstances around it. Like how relaxed and comfortable I am, how deep it goes, how deep of a breath I took before and whether someone's holding my head from pulling back. Under optimal circumstances I could probably get close to a minute before freaking out, but I much prefer shorter bursts of like 10-20 seconds top and giving me a short break to calm down before getting back at it.

Nope.
Generally they're pretty careful when and where they do it if there's a chance they could get seen. They're no stranger to the allure of risk and doing it somewhere public, but they don't just get down and do it in the open. Usually they look for at least some kind of privacy before they have sex. The riskiest and most open they've done was during a bus ride. There it started with Vivi "dropping" something below Kai's seat and then deciding that his lap might be comfier than the bus seat. Then they let the bumps in the road do the rest~
But no one caught them then, they made sure to inconspicuously block all sightlines to what was actually going on.

Though, I guess Stacy did catch them in the middle of it a few times, but I wouldn't really count that since they're all together and no one was really bothered by it. For them it's more like walking in on your roommates just playing a game together or something rather than a "OMG you guys are having SEX?!?!" moment.

Honestly love most pregnancy related kinks! Definitely some of my favorites, and pretty underrated if you ask me.

This of course also includes pregnant sex! Big fan! 💕
Not sure why but it just feels so much lewder and kinkier than regular sex >///>
But it can also make it so much cuter and romantic as well if it's between a romantic couple <///<
Though I understand it's not everyone's cup of tea! There are some aspects that are often associated with it that I don't really like either so I fully understand when most people say they don't like it that much if at all!

I did get this one pregnant Vivi piece for easter last year in case you're interested and haven't seen it:
https://x.com/ViveeEevee/status/1645132430389526532
Will we get to see her knocked up again in the future? Maybe, --probably,-- who knows?~ --Absolutely!--

So of course I don't mind seeing characters pregnant in art. Though I'm always a bit iffy when stuff like this makes its way into SFW spaces. It depends on context and how highlighted it is I guess.
If them being pregnant is the main focus of the art and/or the artist just wanted to draw "XYZ being pregnant" for the sake of seeing them pregnant, then I generally see it as fetish content and that should of course stay in NSFW spaces. Doesn't matter if they're actually nude or not as NSFW is more than just showing certain normally hidden body parts. Context and intention play a massive role in what's SFW and NSFW if you ask me.

You're allowed to ask as much as you want, no need to feel like you have to mention that this is your final time! x3

I'm always happy to answer questions, which is why I have this page~
Feel free to keep them coming!

As for the question, if with "blend" you mean that they fuck their parents, no. Not really.
...
Kai diiid do his mom once but that's it.

And as for whether they know about Vivi and Kai, yes!
They both found out their own way. Diana just kind of just pieced it together. Like for example how she could hear constant bumping noises coming from Vivi's room, the same ones she tends to hear when the eevee brought home her latest hookup...except for her coming home alone. Or how Vivi and Kai started to leave for random "overnight stays with friends" during the same weekends and leave within less than an hour from another.
Sebastian on the other hand found it out the old fashioned way: Walking in on them (Which come to think of it is a bit ironic).
Both parents didn't really approve of it, but saw that Vivi and Kai genuinely seemed to be happy with another and so weren't against it enough as to intervene or tell the other. So for a while it was a sort of open secret. Everyone knew, but no one said anything because they weren't aware that the other parent knew as well. That was the case until Vivi brought home an egg one day and she decided that it was time they told them as she didn't want to raise it on a lie or get her friend into trouble. You see, up until that moment, Vivi blamed her pregnancy on an old friend of hers that conveniently moved away just as she started getting visibly pregnant. He was completely unaware of his involvement in this lie, so she didn't want to have to constantly come up with excuses for their relationship and why he isn't checking in on his child or create an even more awkward situation should he ever decide to come visit one day. She knew this wasn't a way to live and would only cause more trouble, so this was the time to finally be honest about Kai and her.
Them spilling the bean was an awkward moment needless to say, but both parents kind of just...sighed and accepted it as they already knew she was lying about the father.
This would have been a bigger issue if it wasn't for the fact that the both of them are part of the eevee species and only half siblings. The idea here being that unlike that of most other pokemon, eevee DNA is incredibly unstable and prone to random mutation. This, alongside its irregular structure, makes harmful recessive genes far far less likely to become dominant. It's also the reason why sometimes eevee children can look a bit different than their parents, which is the case with Mia having a floppy ear, unique tail pattern and darker fur, as some genes may just be overwritten by mutation (Which can of course also be argued to cause the opposite and make complications more likely, but this is fiction and I call the shots so it doesn't x3). Now add on top of that the fact that Vivi and Kai only share 50% of the same DNA thank to each having different mothers, and the likelihood of them causing any harm with their actions is barely any higher than that of it just happening naturally with regular couples of other species. (As long as it's just one generation that is.)
But their species being the exception doesn't change the fact that it's still outlawed in their world and the act is frowned upon societally, whether it causes harm or not, so they gotta keep it hush hush around anyone that isn't in on the secret.

But to get back to the root of the question again for a second:
As I've said in my last response, Vivi did walk in on her parents, and it did leave a lasting memory and desire within her that she's been chasing to experience first hand ever since. Especially how her father acted. So in a way she does have the hots for him. But obviously given the nature of what would be her request then, their relationship and his personality, she's afraid of how he'd react if she tried uh...seducing him.
However, even if she had the opportunity to "try him", she's not sure if she really would.
Vivi is a little bit of a masochist, something that awoke within her when she saw what she saw, but she does still have her limits. And she isn't sure if hers and his align. But if she wants to fulfill the desire she's had for all those years now, she'd have to give up on her limits, leave the safety of a safe word and let him do his thing, otherwise it'd just be a compromised experience that doesn't actually give her what she wants or answer her question of what it would be like to have been in her mothers position at the moment in time. She doesn't know how far he can go and how much she can handle, but would have to put herself in a situation that she can't back out of if it gets too much. Needless to say she's scared of that.

All right!
Sooo, story time:
Way back before Vivi was born, her father used to be completely different. He was lazy, selfish, loud and rude. Back then, he, his wife and their newborn son lived in what was essentially a shack that was falling apart in pretty much the middle of nowhere. He wouldn't work or take care of his kid and constantly be out of the house. And when he wasn't, he'd just sit on the couch, drink and watch tv while complaining to his wife about every little thing bothering him. One of these days he went to a bar, got a bit too drunk and fooled around with someone. This act ended up creating Vivi, something he'd have to confess to his wife when they found an egg left at their door addressed to him.
Understandably furious, Diana whipped her husband into shape and gave him one last chance along with an ultimatum. She had enough and if it wasn't for the fact that they had not only one but now two children, she wouldn't have put up with all this. The espeon was past the point of caring for her marriage, but didn't want these innocent children to have to grow up with separated parents, so she endured way past the point where others were telling her to stop and leave. This was a sort of awakening for Sebastian, finally realizing what he was doing and that he didn't even deserve this last chance that has yet been graciously given to him. From that moment on he vowed to change for the better, not just for himself but for his family. He wanted to give his children the best life possible and worked hard alongside his wife to get them out of their current situation and into a better home.
Their first goal was to move out of this barely livable home, but they both needed to save up for that. Diana felt awful neglecting her children for the sake of work, but she agreed with her husband that this was no place to raise children and that they needed to leave asap. So they worked together to save enough money to move out of the old, broken shed and into a proper home.
However, because both his parents were busy all the time, Kai quickly had to learn to take care of himself. And soon after he'd also feel like he had to take care of his little sister once she hatched. At least whenever their parents weren't around. Due to this, he built a strong bond with her as he was essentially acting as a third parent for her. He cared deeply for the other eevee and was willing to do whatever it takes to make her happy. Something he'd quickly prove. During a particularly cold and strong winter, the building they were staying in could no longer withstand the freezing temperatures and equally cold winds, especially with the lack of functioning heating appliances and falling apart walls. Kai couldn't bear watching his sister freeze and shiver all day every day, and there was only so much warmth huddling together under a blanket could provide. However, he did learn of the various evolutions that his species could turn into through a children's book, and he also understood that it took a special kind of stone for it. He learned of flareons and the warmth they can provide and figured that if he became one, neither of them would have to freeze anymore. Now, normally since this is an irreversible, life changing decision, parents tend to wait until adulthood before they offer their children the opportunity of evolution. However, he begged his parents for a stone so he could provide warmth to his sister. At first they rejected him since he was still way too young to make such a decision. However, Sebastian, ridden by guilt of his past behavior and neglect, as well as a desire to make things right, bought a fire stone with the family savings behind his wife's back and gave it to Kai in the name of helping Vivi. Understandably, Diana was furious that he made this decision for him at such an early age, but after seeing how happy Kai was and how Vivi was no longer shivering and whining all the time, she let it slide. Probably because she too was feeling immense guilt for having her children be freezing all the time. Kai didn't seem to show any regret for this decision, not even to this day, something that over time eased Diana and Sebastian's conscience.

Fast forward some years and things have changed quite a bit.
No longer did they live in this run down shack. Instead their father somehow managed to land a well paying job at a major company, and thanks to it they were able to afford a nice house in the outskirts of a big city.
Diana and Sebastian’s marriage has mostly been revitalized after the jolteon has proven to have changed for the better and showed his dedication to it over the years. He was essentially a completely new pokemon, and much more like the one Diana fell in love with all those years ago. This did not undo his past however, and he is still scared of it to this day. Scared that his children would end up becoming the same way, lacking discipline, growing lazy and eventually hurting those they loved. Out of fear, he became strict and overprotective. "No going out past 6PM, no going to the beach without a parent, no staying up past 10, and Vivi if you try and reach for the cookie jar again you're grounded for the rest of the week." That along with his unfiltered complaining for any kind of flaw in his kids made him fairly unpopular among them. "Kai, your grades have been going down, you're slacking. Vivi, all those sweets you keep eating are making you fat, you're already getting chubby."
He knew he wasn't being "nice" but he was doing it all to prevent the same things he did from happening again. It was his own way of showing love, by trying to make them become the best selves they can be with the brightest future possible.
Luckily for them though, Diana was there to sort of even it all out. Unlike her husband she was soft and understanding and even kind of encouraged them to not listen to him and live their lives however they wanted (within reason of course). She'd calm down tense situations when he became angry and make him ease up on his rules, stand up for them when they did break them and be a loving and approachable parent when needed. Not that Sebastian couldn't have moments of love, kindness and softness himself, but those were far between. Usually if Kai or Vivi needed someone they could trust and turn to, it was their mother, making her the popular one among the parents. Together the parents created a good dynamic of making up for what the other lacked. Sebastian brought order, direction and rules to help guide them, while Diana offered protection, trust and freedom when things got too much.
Despite Sebastian's behavior, Vivi still gets along well with him. She learned to kind of roll with the punches and not take them that seriously, seeing as her mother was encouraging her to do the same. His rules and lectures became more of an eyeroll that could be safely ignored for the most part. And as she grew older and learned of what he was like in the past, she started to understand him more and forgave him for his behavior. She didn’t like or approve of it, but she saw and understood where he was coming from. That much can not be said for Kai though, as he and Sebastian tend to not really get along that well. They can tolerate another, but Kai doesn't mind barking back when his father gets on his nerves, which happens quite easily.

That is pretty much the family's dynamic and how and why it came to be!
There is of course the mystery of Vivi’s actual mother and that unknown twin of hers buuuut that’s for another time. For now, the siblings are unaware of their existance.

As a final note on the dynamic: Unbeknownst to him, Sebastian has had a major, special kind of lasting impact on his daughter.
You see, one night when she couldn't sleep she came to check in on her parents to see if they could help her. But what she saw that day is something she could never forget...The roughness, the aggression, the intense domination he was showing...The way he snarled, bit and showed no restraints or remorse...Those words he growled she didn't even understand at the time...The way her mother looked pleased despite the things he was doing to her...it was something that stuck with her and a moment she's been chasing to experience first hand ever since. Yet none came close to replicating what she wasn't supposed to see that night...This too most likely affects her relationship with him, whether she’s aware of it herself or not. It certainly shaped her preferences in bed~

1. No, I only really rp with friends and very rarely at that.

2. This is a complicated one that I can't really get into without a massive lore infodump. Let me know if that's something you want!
   But to summarize:

Vivi's relationship with her family (at least those she knows of) is pretty good!
Her mother (Diana, an espeon) is generally soft, kind and loving and therefore earned a lot of favoritism among Vivi and Kai.
Her father (Sebastian, a jolteon) is loving in his own way. But he expresses it by being strict, grumpy and overprotective, which generally doesn't land him any favors among his kids. Though it has gotten a bit better once they grew up and became adults. Still, she can't really stay mad at him.
Out of all of them though, Vivi was always closest to Kai since she could always rely on him and he was always there for her when she needed him.

Unbeknownst to her and her brother however, Vivi not only has a twin brother, but also a different mother than Kai. Though they never met, nor has any of them ever be told about any of that. At least for now...

Vivi wouldn't be content with just one super power, she'd want to be an evil dark queen, ruler of everything and servant to none, so she'd pick all of them. Buuuut if we limit her to only one, she'd probably choose teleportation so she wouldn't have to walk anywhere like the lazy vee she is.

Stacy would take the ability to breathe underwater to make up for the fact she didn't become a vaporeon.

Rikki would pick time travel, specifically to be able to undo past mistakes.

Alex takes super strength. The less people that are able to stand in her way and the more things she can brute force her way through the better.

Zoe would pick some sort of healing and calming powers to heal others and instantly calm them down when needed.