Steak and stake are homophones. A steak if a piece of meat. A stake is a pointed piece of wood. In folklore, driving a stake through the heart of a vampire was the only want to kill it. A carnivore, or someone carnivorous, is someone that eats meat. The joke is playing on the word "steak" because it sounds like "stake" and both apply to this joke and make sense.
Legends tell us that a weakness of the carnivorous vampire is a steak .