It depends on the Vampire. Vlad has no trouble drinking from anybody, anytime. He probably wants witnesses because he wants them to be afraid.
The Vamp family I have created, the Constantine's, on the other hand, all are from a Vampire Clan that has refused to drink plasma from unwilling donors for centuries - which is why Vlad despises them. They usually go for plasma packs or, in a pinch, will drink from a willing donor. They probably consider this something that should happen in private, now that I think about it. Unhappily, the mother (whose name escapes me) was out on a date and needed a quick snack while I wasn't looking. I went with it, and she is now wracked with guilt because she fed off a hapless sim. Even worse, she has acquired a taste for plasma, and fights her urges. No one else in the family knows about this weakness.
