It's more of a slow burn!!

Vlin was (is?) Ishgardian and has met Haurchefant before the Calamity (weren't a couple though, but there were some feelings that were unspoken), but "went missing" and lost his memory, finding his way to Gridania around when ARR starts. So Vlin had a faint memory of who Haurche is before MSQ introduces him, but didn't remember until it was too late.

I have this scenario in my head that Haurche at some point, a week or so before The Vault, asks if Vlin remembers anything from the past. At first he says no, but in a good 'ol "say their name" fashion (since Vlin is a name he made for himself, Vyramaux is his 'real' name, funny looking as it is), a vague Echo memory plays from Haurchefant's perspective. At this point, Vlin had some feelings but was confused because "Idrk this guy? But I feel like there's more to him? And he seems really nice?" Then after the memory (Vlin still doesn't remember much, but it's enough of a push) they get closer to each other in a shy but sweet kind of way.

Then y'know, The Vault :^)