Vaults are special rooms which occur in Zones. They have a predefined layout. They are typically full of monsters (often with much higher levels than the zone would normally permit), and usually have several valuable items on the floor. The types of vaults which may appear in a given zone are defined by the zone with some that are shared among zones.
Items in vaults do not decay when you leave the level and return. Items outside a vault may decay (this is defined per zone). This means that any vault can be used for permanent item storage. This can be convenient during the early game, before you have acquired the Sher'Tul Fortress (which is explicitly designed and sanctioned for permanent storage).
Most vaults may be flipped or rotated (the permitted geometric transformations are defined by each vault), so you may find that your vault is a mirror image or a rotation of the layout shown.
Teleportation to a vault tile is generally not possible; however, teleportation from a vault tile to a tile outside the vault usually works. Please note, many vaults have an outer border which appears like the rest of the zone, but which is still considered part of the vault, and teleportation to those tiles may be restricted.
Most vaults have undiggable walls, possibly together with diggable walls; the appearance of the two wall types is identical, but looking at a diggable wall (for example, by putting mouse cursor over it) will show "diggable" descriptor.
Many vaults (particularly the larger or more dangerous ones) have a door which prompts for confirmation upon opening. There may also be regular doors in the same vault. Monsters in a locked vault are inactive and will not teleport out, summon creatures, buff themselves or move around until the seal is broken.
Vaults can't be mapped with Visions talent or abilities that duplicate it, but Arcane eye can be placed and can spot creatures within even a sealed vault.