[*]It is unknown how long it would take a vampire to die from not eating, but it can be inferred that ~50 is when a vampire would potentially enter a comatose state. This could prolong the time it takes to reach death.
I'd like to emphasize here that it is just pure speculation that vampire would die at some point without feeding. This is just my feeling, but it doesn't make sense to me, how someone who manages to survive for 50 years without any food, could die from starvation.
[*]Behaves as a disease in every way, even called one, but is also a Divine curse as well. (Though I'd personally consider it a viral infection)
There is no doubt that it starts as a disease, the thing is what happens next.
[*]Can reproduce sixually, producing offspring. Whether the offspring are truly half vampire and half other is unknown, as we only have one case, and the only apparent difference are aesthetic.
Grey prince seems to be an exception, a divine intervention from Mara perhaps... there is no other example. Also the ability to reproduce doesn't have to contradict that they are undead.
While it may be gameplay mechanics, I do find it interesting that turn undead works on everything but vampires(With the implication being there's no bound soul as there is with any other undead, implying they never died in the first place).
I think it is gameplay mechanics, there are two reasons why I think so: 1. Vampires are classified as "NPC" other undead are classified as "creatures". 2. There is this passage from Vampires of Vvardenfell:
...The "ash vampire" of Ashlander legend is not undead. Sorceries and blessings affecting the undead reportedly have no effect on these creatures. I know that it doesn't directly say that magic against undead works on vampires, but why mention that it doesn't work against ash vampires then?