To what extent do things respawn? I've pretty much determined that zombies in a necromancer tower are infinite if you leave and come back (even if you come right back). It also just happened with a vault. Also, what is the point of the vault slabs. I read this one, but nothing directly happened. All it said was something like "I am X, and with this I am bound to this world".
Vault Slabs are the keystone that Demons use to tie themselves to this world. If you happen to go the Dark Fortress where the Demon that made that Vault is, you can then use what you learned from the Slab to either banish him from this plane, or submit them to your will. Good luck finding where they are without Legends Viewer though.
Oh, and the Undead and Vault Guardians always respawn eventually. Undead come back the moment the area unloads, not sure about how long it takes guardians to come back.
I read that they are the key to becoming a necromancer, you need better reading skill going by the wiki.
You are thinking of Necromancer Slabs, Vault Slabs are very different. You also only need a Novice skill in Reading to be able to read anything and everything.
The population of necro towers is tracked, the population is just really high. So you can kill em all eventually. Vaults are respawning likely for gameplay reasons since guardians are like demons.
You can verify this with legends viewer.
Nothing else except demons "respawn" since df tracks and ticks down populations of everything including wildlife regional populations. Df tracks historical figures and entity populations, but since the game would die if it had the full population in a site it just grabs people from entity populations as needed, this results in something that looks like respawning in some places with high populations like cities and towers but what is really happening is that it just loads up and ticks down "entity populations" so it is possible to kill them all it just takes a lot of leaving and coming back.
This is even harder now since the population is activly simulated as you play now, due to "world activation" so populations can now tick up aswell as down