Is it the vampire in Verdistis or the one who kidnapped the boy?
In any case => I'd try an area spell => elemental strike, poison etc.
Could you try this => save under another name. Exit the game and then restart/reload this, maybe he has moved a bit? This normally works for the char, not sure if for NPC as well.
Reviving dead NPC, click hereI'm not sure, if Raze would recommend this for a stuck NPC as well - but this is all I can think about now.
Kiya