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.
Vault 15 in FOnline 3
In FOnline 3, entering the location puts you in battle with raiders instantly (although its "green" location). Vault itself is an infinite dungeon, spawning waves of raiders. Every few waves, locker inside will fill with various loot.
This dungeon is an easier Vault 14 version.
Vault 15 is located at Grid 39:27 (due east of Shady Sands).