You can edit the testset for testSet_ReasonToSlay_HungryOrAngry_Actor and tweak the vampire thirst level so its more available (it's set to a threshold of less than or equal to -45 now, vampire thirst runs from -100 to 100). In terms of the mixer taking priority over other mixers, it's about as tuned up as I can get it to go.
There are also some tests to make sure vampires only drink in appropriate settings - if they're the lot owner, they can drink any time after dark. If it's day time they have to be in the house. If they're on your lot, they won't generally drink unless your sim is alone. The exception is if they're a doorbell ringer and trying to sneak in, or one of the vampires doing a walkby (and actively hunting.)
