Vault 15 is one of the Vaults built by Vault-Tec. Like Vault 13, for example.
The situation in Vault 15 led to a schism, and all the inhabitants split in four groups. The Jackals, the Vipers and the Khans turned into raiders that only wanted to kill everyone in the wasteland ; while the fourth group founded the Shady Sands that would later become the great and powerful city of NCR.
On the very last floor, there is a computer in the room to the left. Use Science on it and you will obtain Electronic Parts. There are a number rats, pigrats, and molerats that respawn every so often. You can also find Medical Terminal inside Vault 15.
Vault 15 is located at Grid 39:27 (due east of Shady Sands).