I just finished exploring a vault. Then decided to wait 1 hour, as last time that got me teleported back to the surface. Seems like it procedurally re-generated, and now I'm closed in by vertical bars.
Also, before that, I found the slab. Stuck in a wall. In the first open chamber part.
The vault was a bit branched, not that linear. I had to open one set of vertical bars to get to a lever to open another set. Which lead to an open room with an elevated area in the middle of it. I guess that's where the slab SHOULD have been.
In addition, I ventured in with a previous adventurer who could bleed (but still had steel skin). He died when he jumped off the top of the structure he got teleported to. Nevermind that. Thing is, he found minions. This new adventurer didn't. No enemies whatsoever.
Pic of the slab in the wall before the structure regened.
I'll see if I can get out somehow.
EDIT: It's not generating again. I don't know if it didn't originally, but I'm sure the positions of the bars weren't there when I came in. Now I'm stuck though. Guess I'm the secret of the vault.
EDIT2: Had to abandon. I'm in a new area. Retainers of Ud (animals), Enforcers of Ud (weaponlords) and I've just found a "Protected Monster". How strong is the last one (which is one of those unique enemies)? It managed (with a lucky hit) to kick off one of my fingers. Mind you, I'm playing what is practically an Adamantine Colossus. Its other attacks glanced off, but all I can do is fracture his parts. These things are serious business.
I can't break the grasp of its wing on my thumb. Even with natural legendary skills and extreme stats and size. This thing is crazy and not dying or letting me go.
EDIT3: Summing up what I've found: Vaults have 3 types of enemies: Animal-type, Human-type, Monster-type. All procedurally generated, obviously. They are increasingly dangerous, though I've found animal-types to be somewhat more resilient then human-types. Human types have items of divine materials; a weapon, a shield and some clothing (divine cloth). They all weight >1 urist, and I believe elsewhere it was already stated that they are practically adamantine. Dangerous. I've actually crushed things with a shield, so I don't think the mace/hammerlord version should be underestimated.
The monster-type enemy is the most dangerous from what I've seen. The one I fought was insanely strong and tough. I'm going to say that breaking their grabs is damn near impossible, since my modded char couldn't break its hold (100x human size, extreme stats and skills, made of full adamantine). I'd advise evasion if possible.
Vaults seem to consistently penetrate the first cavern layer. Animals don't get inside, but you may encounter (giant) cave spider silk. Try to avoid it, as fighting the enemies as is will be challenging.
Finally, there are plenty of falls you need to watch out for, but thankfully, most of the time there are no enemies near such areas, except in one of the first few rooms of a vault.
Oh, and the slabs seem to do nothing. They are always located on an elevated platform somewhere near the end of the vault.
Pic of the layer where I found the slab. As a bonus, it looks like a toilet.