I assumed they could breathe voluntarily but not as a reflex.

How would they make poor servents though?

Are vampires half daedra? I remember reading somewhere that said vampires may be half daedra, half undead.

Probley harder to control than a mortal would be.

Yep. My thoughts. They would rebel. Vampires are Molag's creation, I doubt they would want to submit to another Daedric Prince as slaves.

How often are zombies created by a disease yet they're undead?

A vampire does not have a disease, they even have 100% resistance to disease. Being a vampire is the end result of a disease, once you've undergone the transformation it can't be cured simply by a simple spell or visit to the temple. Only a rare few no the rituals necessary to return life to an undead body.

Good points.

With how I look at them, I do not believe that Vampires are dead. They need to feed, they breathe, they sleep, and their flesh doesn't rot.

As for how in Daggerfall, when contracting vampyrism, you "die" and later wake in your crypt; Modern medicine would describe that as a coma.

No. It is death. Read all the ingame books and dialogue in the games. The vampire cure quest states the vampire is dead. When you use a detect dead spell in the game, vampires glow rather than regular people.

Most importantly vampires don't show up on detect life, but they do on detect dead.