Vault 24 is one of the Vault-Tec vaults.
The vault's original purpose in the vault experiment is unknown. A Vault 24 jumpsuit is the only trace of the vault in the game, which can only be accessed by console commands.
Vault 24 is referenced in the game files of Fallout: New Vegas in the form of a jumpsuit for the vault, but was not included in the game.
Behind the scenes
There is currently no mention of a "Vault 24" in any official Fallout universe resources, including potentially non-canon sources such as Van Buren.