Well... I was trying to ignore the most problematic aspects of the story but I've got to say, showing the popular and bubbly life, from when her thoughts and feelings were her own, of the girl who just got brainwashed into being in love with Ruruna is really twisting the knife in the salted wound.
Moral quandaries that sour the fluff tend to be written around in series like this. I'd say there is like a 90% chance the girl is not actually enslaved and is playing along with it because our vampire lead is so adorable.
It feels weird that she calls her "Master" rather than "Mistress".
That sort of stuff happens a lot in non-English media using gendered English terms like that. I'd say it's kind of a trope at this point that female characters get called male gendered titles in manga and anime. Could be part of the general trend towards gender neutral terms in English, or, more likely, a product of Japanese people not being fluent in English. I'd just get used to it as a quirk of the medium.