One of the best ideas to come out of the over top 2E Forgotten Realms, in my opinion, was the "illithid lich", or "mind flayer lich" which I like better as the name. It's completely nuts. For a vampire lich, I'd use similar approach, where it is not something that can be replicated but rather is a case of magic messed up. It's more superhero origin story than a standard creature, so you need your fantasy equivalent of the radioactive spider bite. You may have more than one, but it's a finite set from one unique incident.