CRACKS MY KNUCKLES SO HARD. YEAH. ALLOW ME TO ELABORATE HERE.
The overall arcing conflict (imo, ofc this is my interpretation) for trigun is less humanity verses plants and more a difference in a sense of belonging and loyalty. Nature verses nurture. Knives, seeing the atrocities that humankind is capable of towards his people (and his brother) does everything in his power to ensure that he wont lose them, and wont have to lose any of his sisters ever again, even at a great cost. Vergil, who was left behind after the attack on his family home, seemingly alone and abandoned (i have played the games but yet to read the manga, but have seen a few analysis vids so. forgive me if im off the mark) thought he lost everything. He wanted (in my opinion) power not for the sake of being powerful, but to stop tragedy from happening to him again. If he was the STRONGEST then hed never have to worry about being hurt again, and i like to think that his goals only strengthened once he realized dante was okay and alive, even after being struck down by him personally, and thats why he planted the Qliphoth. To consume its fruit and become all powerful so he wouldnt have to be hurt again. Not an exact parallel, but Knives was collecting his sisters and strengthening his own powers to protect them, just without his own personal sacrifice of humanity like Vergil sundering V from himself in 5.
ON THE FLIPSIDE OF THAT we have Vash and Dante. Vash, who was hurt by his past and forced down a path he probably had no choice but to take, found healing and belonging by embracing humanity, even when they didnt trust him and used him. He was in opposition of his brother on the principle of "human lives matter" (simplified) at the cost of something precious to him (plants). The themes get a little wobbly here because there isnt quite the same stakes 1:1, but, bear with me. We even get the whole "killing my brother" in tristamp that parallels with dante as well. Dante's perspective on this though, i think, aligns a lot with Vash. For Vergil and Knives, Power is their liberation. For Vash and Dante, freedom is their power. Dante is naturally gifted, the opposite of vash, and therefore he doesnt have to try as hard. he ended up in the care of humanity, embracing his humanity, and shying away from his demonic blood unlike Vergil. (like vash and knives, too) but he craves adventure and excitement in a way that leads me to think thats how he copes. He cant stay put, he doesnt like to be tied down unnecessarily or recklessly, so hes always trying to find something to do (devil may cry) until his hand is forced and he has to intervene with his brother (temen-ni-gru, the Qliphoth) and then its just downhill from there.
I feel like both pairs of brothers feel some sort of responsibility for their predicament, be it positive or negative, catastrophically self destructive or nihilistic in a way, there are a lot of similarities in their base moral compasses that ive really been enjoying dissecting. This could have been much more thought out but its 7am and ive yet to sleep, lol. I like their parallels a lot.