Beatrix is the only vampire you can become a thrall of. Nashara is an old test character left from when I was experimenting with RPG Maker to learn the ins and outs of the engine (I was trying to figure out what limits are there for unique events and mechanics during a fight, hence why she could "enthrall" you). I think the really early versions of the game had a few more bits and pieces like that, but I mostly removed them over time.
P.S.
Glad you are enjoying the game! :)