It's not the first time a "vampire" has been pitched as a job concept, and almost positive it wouldn't be the last. The only real issue is that it would require the protagonist (the player's character) to find some way of committing unspeakable horrors without being seen as "evil", especially when Black Mage is already perceived to be right on the edge of that. As for transformation, I know for a fact that this wouldn't be the first idea to suggest it. I did it before making a concept of "Sorcerer", although to bring it to multiple roles would either mean changing itemization (which seems far too unlikely) or having to rubber-stamp it with the L-word as a job.

The closest thing I could forsee is a job using a transformation effect these days is as a cooldown ability, but likely nothing beyond that. Whether or not it's anything like a vampire, I doubt it unless it does a really good job changing our understanding of what a vampire is.