So I've thought about this a lot since fierre showed up and this is going to be a pretty long explanation with a lot of points but I think it makes sense. I've broken it up in sections to try and make it easier to read. Please bear with me!!

I have this running theory that the city in the dungeon and the mother of graves is the source of vampirism, and the mother of graves, which I believe was confirmed by xrn as being a type of creature she had fought on rhir, came there during the creler wars and infected the city of graves. It makes sense the mother of graves is a rhir beast and vampirism came from there because we know demons suffer a blight that changes their nature and crelers could spread world wide so why not other things?

Think about it. We know the dungeon is a containment dungeon. It's containing the city at the center, the city of graves. The mother of graves is in a huge crater in the center, implying it attacked the city or something and was bound there. Meanwhile the entire city is filled with skinless Drake's that, other than slightly enhanced physical abilities, don't seem as powerful as the other monsters. They would be the populace of the city of graves turned vampire by the mother and trapped underground with her. We know vampires are weakened over time by not feeding. It would make sense that they wouldn't be much stronger than normal Drake's after all this time stuck without food. And once they realize someone is there the entire city of them goes after that person and tears them apart. They're mad from starvation. It also explains why their numbers don't increase or replenish.

Meanwhile the rest of the monsters, the legions of armor, the flesh worms, the undead, etc. Are all seemingly more impressive but none of them are actually IN the city. And the armor, probably the most powerful and definitely the most numerous are focused patrolling around the city. Vampires can't eat armor. Plus the living monsters can't see each other due to the water that makes them invisible to each other. Yet the rhaskgar had no problem seeing the skinless Drake's.

The rhaskgar themselves were regular gnolls that were forced underground and were changed somehow. They also practice a nasty blood magic ritual where they eat gnoll hearts that makes them stronger and faster and smarter. They also hate the daylight. That's very vampy. They're also the only ones that even try to raid the city. That's clearly where they learned about the ritual, or at least it makes sense. That would work if the creatures in the city practiced extensive blood magic, which is usually associated with vampires. Blood magic that may have been learned from the rhir beast bound I the center and slowly corrupted the cities people. Plus we know something about the dead in that city was able to slow down kasigna in the afterlife even though she's the literal goddess of death and consumes souls with a touch, so there's more to them than just being dead souls. Only rhir and khelt have been shown to be able to resist the gods like that in the afterlife, rhir by virtue of being even more horrible than them.

Consider the guardians also. Snatcher takes people's heads. Skinner strips off people's skin. If stripping a vampires skin makes it more vulnerable to sunlight, a fair enough assumption I think, then both of them seem focused on killing in ways that happen to be traditional ways to kill vampires. Stalker was killed by snatcher but he was also the only monster we've seen inside the city besides the rhaskgar, at least his corpse anyway. What if snatcher killed him because he entered the city and became infected? Doesn't seem like a stretch. And neither of the other two are ever seen in the city even though both have clearly been there. They both seem to focus on killing outsiders also, even though we know BOTH could still see the other monsters in the dungeon. They're there for containment and preventing outside interference.

The big monster pits the shield spiders and moths escaped from would be the nuclear option and the trap rooms by the gate an attempt to keep them locked in if all else fails on top of keeping everyone else out.

It would also make sense that every vamp we've seen has been human. Any drake vamps that escaped would have fled north if the walled cities united enough to bury their city and build a death dungeon around it. Then they would have infected the humans when they invaded izril before being wiped out by houses like byres. That also works with my theory about this happening during the creler wars because we know that house byres was already present at this time due to that silver dragon and there was at least one ridiculous necromancer running around(the putrid one) so necromancy would have been at a entirely different level then

It also explains why Liscor was built where it was but no one even knew there was a dungeon there until recently. It's a strategically important spot for izril, enough a walled city used to be there. Someone eventually came around years later and built a new city to secure the flood plains. But after the city of graves was buried and the dungeon built all record of it would have likely been destroyed. It's a containment dungeon after all. You wouldn't advertise it's location. You'd bury it. Just like the mother of graves.

Anyway, that's my theory!! Thanks for hearing me out.