powerful lich
I know that he is supposed to be some type of lesser / demi deity, but really, to the typical 1 - 5 level NPCs, once you reach the higher levels of lich (or even demi lich), the being is just as lethal as if it were a demideity. Dead is dead, after all, and a demilich (or even a lich, I think) can do worse than mere death - sealing the soul within some item (usually a gem, at least for demiliches) for slow torturous consumption. Really, if Vecna insisted on being called a deity, I can't see those under his terrible rule arguing - or even believing - otherwise. Dead is dead. Sealed in a torturous afterlife is sealed in a torturous afterlife - whether that be in another plane or in a gem.
So I suppose I side with a variant of Demilich rather that true demideity - even if Vecna insists that he is a demideity. In Ravenloft its not as if there are other deities around to argue with him - at least not in his domain. Others may exist of course, but as with all dark lords, they will be sealed each within their own domain, having little or no direct contact with each other.