I don't know how I would convert that model to be more like a vampire, but I think you could use him as he is right now. Just give him a more undead paintjob. The older fluff says that Blood Dragon vampires sometimes ended up as assassins, and this guy looks just right for that.
I would swap the broken elf architecture on the base for a gravestone of skeleton, or remove it entirely. Swap the left arm for one with outstretched fingers and have an ethereal skull rising up toward it to suggest necromancy. I would also suggest sculpting the vague outline of faces on the inside on the cape, similar to the ones in Vlads.