I think making it gain the vampire ability is a bit OP. Imagine you steal a 21/11 golem, you heal 20 each turn?
better than stealing a golem and making it keep grow every turn? obviously it helps mono darkness some but I don't think they should be stealing creatures and using their powers... and it fits the elemental theme a little better: It is betraying it's owner because it's now a vampire. It also means stolen small creatures die after a few turns.
And let's be real... the OP part of that is not that it becomes a vampire but that someone spent 6
to steal your 21+ attack creature