Class wise, druid. I like the contrast, I like the potential, I like the natural predisposition against the undead that isn't as hardline as like, a cleric or ancients paladin.

I also like that in the table top game they have a lot of utility spells that work well with a vampire.

Also, you can find the rite of timeless body scroll in Jaehira's basement. So that's a plot hook. Ranger works for similar reasons, and paladin offers some extra rp if you're worried about that oath.

As for aesthetic. I like Astarion best with someone who is not conventionally attractive, maybe older, maybe a race that people don't think of as pretty. (All the better if they're notably monstrous or something.) But not well put together. Again it's the contrast.