I have noticed two things: 1. Plasma packs don't provide much plasma. I think they fill the thirst bar up about 1/8th to 1/4 of the way. I don't think the plasma fruit is much better. So snacki...
I think it would be too easy, if the plasma packs filled up the thirst bar more than the currently do. My vampires just always have at least two frogs in their inventory and breed them frequently. That way they can always make plasma packs on the go.