Vault 21

 is one of the Vault-Tec Vaults, located in or near Las Vegas, Nevada. Its purpose in the Vault Experiment was gambling, and all conflicts in the Vault were solved through a toss of the dice. Mitchell comes from here.

Layout
The Vault has been converted into another casino. It is located on the Strip, and is lit up like a giant Christmas Tree. It is also a hotel, because on the GameInformer screenshots, the letters HO can be seen on the sign

Appearances
Vault 21 appears in Fallout: New Vegas.