ToME: the Tales of Maj'Eyal

Are these special areas found in each dungeon or are they actually a random chance? I think I have found a few (semi large diamond shaped structure with many rooms inside with loot and hard enemies in) not sure if these are vaults but yeah, how do these work?

Vaults are randomly-inserted rooms with extra dangerous enemies, special ability restrictions (to prevent the monsters from escaping), and fantastic loot (typically). Nearly all have a door that will ask you to confirm before you open it, much like the stone in Bill's lair (if you've found it). Sounds like you've found a few.

Some vaults have special mechanics. Most have a theme in mind. I recommend leaving them alone for a while if you aren't sure what type of vault it is, or if you aren't confident in the survivability/escape options of your character. Just a warning, you can't use teleport abilities inside a vault (unless you target outside the vault), and most vault walls cannot be removed (digging).

Ahhh good to know, I just struggled through one with my level 14 dwarf berserker, level 30+ enemies hurt :/