Vault 21

 is a Vault-Tec Vault, that was once hidden under Las Vegas, Nevada. It has since been converted into a hotel.

Background
Its purpose in the Vault Experiment was gambling, reinforced by having only compulsive gamblers admitted as Vault residents, and all conflicts within the Vault were solved through gambling. Mitchell comes from here. At some point, Mr. House decided he wanted to own Vault 21, so he bet for it. He won, and later converted Vault 21 into a hotel.

Layout
The Vault has been converted into another casino and hotel. The Vault is actually advertised with a big sign, unlike all the rest. Its supposed Overseer is Mr. House.

Appearances
Vault 21 only appears in Fallout: New Vegas.

Behind the scenes
Vault 21 is a reference to the card game "Blackjack", where a player has to have the highest sum of card numbers without going over 21.

Krypta 21 Убежище 21