Like other comments had said before...MC can still survive if she plays the S&M or Kink roleplay card excuse...Or if the Nun is the one that actually misunderstands the whole 'Master' term comment and comes to her own false (Technically false but almost kind of maybe correct) conclusion about potential kinky stuff.
Improvise adapt overcome MEME!!!!!
Vampire blood on her nun outfit....Look I know Vampires love drinking blood but if your food has touch the floor for more then a few minutes...Would you still eat it [Yes don't answer that...I know that there are humans that do so but they often get sick 30% of the time...].
Plus the blood on Nun is probably dried out if it been on the nun more then 10-20 minutes being exposed to the air and sunlight (Plus I question if consuming blood of a fellow vampire if be considered cannibalism??) XD
So I am wondering when the Nun officially joins the harem...Hmm I be betting perhaps maybe in 4-5 more chapters after some troublesome misunderstandings and action chapters, could be longer though.