User blog comment:Zippertrain85/Vampires are NOT undead in TES/@comment-6336669-20140729040611

If I remember correctly, I think it's stated that undead are undead due to their souls being caught between the afterlife and Nirn, and if Vampires were never "Killed", their soul could just be partially trapped in Molag Bal's realm, and their bodies, which are still functioning due to never actually dying. It has nothing to do with flesh being dead, which reminds me of the Gatekeeper from the Shivering Isles. They're not "undead" because they never had a soul in the first place, it's just a bunch of flesh thrown together and inducted into life by magic, potentially with help from Oblivion, considering they could be considered Atronachs, which are Daedra.

Generally, I don't know but I'm pretty sure being "undead" has nothing to do with having a previously dead body.