I mean... maybe they can control it? in most vampire myth, a vampire must drain their victims entirely off of their blood and inject some of the vampire's own blood back(the new vampire would also only rise after three days, but eh), perhaps Vladarnic was just doing a very rapid version of this due to his immense size.

As long as she could show restrain to never drink too much to the point it could induce anemia or other health issues, I wouldn't mind that much? at that point it is more like understanding your partner has special needs, and the bigger issue is how often she could do it without causing problems.

(some very quick research shows an adult person can donate about 350ml of blood at once, the body is able to restore the lost blood volume in roughly 24 hours, and it takes two weeks to restore the red blood cell count, so not that often)