I can speak only from my own experience, so it will probably vary with the rest of you folks. The only time I've seen the count(ess) autonomously bite is when they had a high level relationship with the victim, no asking by either party, and it could happen anywhere. If my own sims were vampires, they could bite anyone they had a good relationship with, but it was under player control. No asking first, either. Unlike TS4, biting is to transform into vampires, not drain the victim's blood. Have checked the book, and it says that once a sim has been bitten, the master/townie vamps will autonomously bite sims with whom they have a high relationship up to 10% of the population. Once that cap has been hit, only player controlled vamp sims may continue to bite.
Concerning needs, they do not drop at night (the window is 7PM - 7Am). During the day they fall faster than those of regular sims, even indoors or in cars. Outside, the needs are in free fall. Best solution is to get your vamp sims into a coffin as soon as it gets close to sunrise.
Another source of vamprocillin, if the site is still active, is Parsimonious. They have bottles of elixirs for all the exotic states, both to become and get out of. Tons of other cool stuff, too.
