I just used legends viewer to try and find out more about these vaults, but every time I try to view the details on them, legends viewer refuses to open ANYTHING afterwards. It seems that other than it being a site and having population, it simply is not a part of history. I can ,however, use legends viewer to locate them, I simply have to use the map and search the sites until it I find a vault, and I can confirm what was said previously about the population count. 76, in EVERY single vault. 50 of one type, with the titles Slave, Attendant, or Being (of ___). 25 Slayers or Spirits. 1 Behemoth or Doom. I probably have missed loads, as this is just what I've seen out of the like 4 vaults in this world. You will probably also be interested in the population count of the Dark Goblin Fortresses. Here's the counts for just one. 2898 goblins, 1550 trolls, 94 humans, 42 dwarves, 20 human prisoners, 15 dwarven prisoners, 6 elf prisoners, 4 elves, and 1 goat demon (and a partridge in a pear tree). But seriously, over 4000 enemies, that's crazy. And the boss is of course always a randomly generated demon, I see a blind one, a goat, and a pale pink one. This is odd, I can only find three fortresses with demons, but there are 4 vaults. Perhaps this means one vault does not have a slab.
EDIT: And even stranger is the extreme amount of artifacts in the form of books... correct me if I'm wrong, I've only been playing for about two months, but weren't there normally like 3-4 different artifacts before, just for granting necromantic powers? Now there's 174 artifacts, 169 of which are books, all with titles. They don't seem to do anything, but they have a number of pages, and they seem to be mostly essays and autobiographies. They are all written by necromancers, and stored within towers it seems. Does this mean we can read them? What purpose do they serve, like why would I acquire the autobiography Humans: Fact, or Fiction, written by the necromancer Zega Spiraledmothers? I mean, they are probably simply fodder to avoid you finding the book that grants powers, but its freakin weird. I look forward to shuffling through a necromancer tower reading every page in order to find the real source of power.