Most Vamp Sims will not automatically go for food on your lot, I leave out lots of plasma fruit to help cut down on thirst build up, they usually go for those, but I have pics of vamps dying of thirst 2 feet from a fruit, so it's not 100% reliable. Some Vamps will raid your fridge for plasma juice, but not many, I think it may be moochers who do that, not sure there though.