The Black Library of Drakenhof is apparently a huge dome shaped cavern carved deep beneath the castle in Konrad Von Carstein's time, so I think it should have survived pretty intact. It has stalactites on the ceiling, so it's been carved out of solid rock.
Just checking Night's Dark Masters and I found this gem:
Count von Sangster is also famed for his scholarship, amongst both mortals and Vampires. It is said his great library is rivalled only by that of Mannfred, and Sangster's recent History of the Empire is considered to be the greatest and most complete ever compiled. So much so that Human scholars will gladly risk the association with the author to read a copy—and as he lives in Nuln, access is not difficult to arrange.
…
In some cases, Vampires don't even need to step outside to hunt, their minions hunting for them or their victims eagerly lining up to give their blood to their masters. It is not uncommon, then, for Vampires to become recluses or shut- ins, addicted to the safety and comfort of their familiar walls. Count von Sangster is said to have never set foot outside his great library for over seven hundred years.
Has there been a canon vampire living peacefully in Nuln for seven hundred years, at least semi-openly?
I suppose it's possible. If he pretends to be an elf then the age could be explained, and if he's good enough with magic he could manage to feed without being caught if he can lure minions in and has the control not to drain them to death.
Still…