So, recently there's been a discussion about the statues of Vampires, if they are considerd "living" or "undead", personally I don't feel they are "undead" and I will explain why.
First of all, based off of the fact Vampires do bleed, they must have some form of circulation and heartbeat, they also have a digestive system, since they do eat, and there's evidence to show that males can produce sperm. How would an undead creature have all these things functioning? Doesn't make sense, epseically since Drauger the other undead in the game have none of these features, they are only powered by Necromancy, but are pretty much rotting. Vampries on the other hand don't seem to be this way.
Another point which is quite obvioius is that a player is never actually dead before they become a Vampire, contradicting any idea of them being undead. Since by defition something needs to rise form the dead in order to be undead. So, this in it's self I believe is proof that the Vampires cannot be undead since it goes against the very definition.
Now, you might be thinking how Vampires are labeled undead by the games they are in, however this may not be the case let me explain this. A possiblity is that this could be a reference to how in other forms of media Vampires are really undead, since there is not a single instance of lore that says a Vampire is undead in the Elder Scrolls (quite the opposite). A user actually gave a theory which also makes sense, it's that Vampires may not be living or undead, but rather something in between. Like, they're alive to have organs and whatnot still work but they aren't the same as a regular Mortal. This might also explain why they are considered undead, since if they weren't only Drauger would be used for "Detect Dead" making the spell almost useless in many areas, in order to make the spell more helpful Vamps were put into that category.
So, this is my opinion on the matter, if you disagree, feel free to share it in the comments below!