Well Anne Rice's vampires don't use the bathroom, but they don't have sex either. Her vampires are rather stupid in concept, when you think about it. I mean, their penises are permanantly erect - stupid.
Vampires in general don't make much sense either, but Buffyverse vampires might as well be alive. Their blood circulates, drugs can effect them, they can have sex, I do think they have been seen eating even when they don't need to...so I don't see how they can't use the bathroom.
Blood can't disintegrate inside them. Some of it has to be used to nourish them, but stuff has to go somewhere, and it can't scientifically just become nothing. It's matter, which can't be destroyed. If they never used the bathroom it would have to be stored inside them, which means after a short while you'd think they'd get fat.
Also, when you think about it, if a vampire is truly dead, they shouldn't be able to have sex at all. And even if you "shoot blanks" you still need body fluids for that, and if a vampire has body fluids, they're not very dead.
The only explanation for THAT would be that the bodies aren't DEAD but that the person is dead and they're body is serving as a host for the demon that takes them over, in which case that body should still have to excrete waste if it consumes anything.
Ooh sorry for ranting, I was sort of thinking that all through in my head as I typed. 🙂