Do you have a recent save you could reload and redo if area effect spells do not help? In various tests trying to kill friendly NPCs, I found Poison Cloud worked well, where they were immune to other attacks, so maybe that would be effective against enemies stuck in a wall.
The technique in the post
Josephine herself stuck in the wall might work for this. Let me know, and I'll look for one of my saves at that point.
Perhaps teleporting past the wall yourself would allow you to attack the vampire? (especially if area effect spells force him further back)
Teleporting Anywhere, via hex editor
