Cast this spell only if you control two or more Vampires.
Destroy target creature. You gain 4 life.
Whether you control two or more Vampires is checked only as you try to move Feast of Blood to the stack as the first step of casting it. It doesn’t matter whether you still control two or more Vampires as you finish casting Feast of Blood (in case you somehow sacrifice one to produce mana) or as Feast of Blood resolves.
If the targeted creature is an illegal target by the time Feast of Blood resolves, the entire spell doesn’t resolve. You don’t gain life.