Unfortunately, we were crowded and life was very bad. There was a schism, and many people left, taking with them the best equipment. Still, some of us tried to stay in the Vault. But then we were attacked. I was hurt, and I ended up here. Now I try to help people..."
Like many of the vaults, it was part of Vault-Tec's grand social experiment. The Fallout Bible (by Fallout 2 / Van Buren says the experiment in this vault entailed that it was to stay closed for fifty years, populated by vault dwellers of extremely diverse ideologies and cultures. The situation caused by the experiment led to a great schism, during which four groups ultimately left the vault to brave the wastes during the spring of 2141. Three of these groups became roving gangs of raiders: the Jackals, the Vipers, and the Khans in the winter of the same year. The fourth group settled down a few miles west of the vault and founded the village of Shady Sands in the spring of 2142.
While the three gangs harassed the village and each other over the years, Vault 15 itself gradually fell into disrepair. By the time the Vault Dweller came across it in his search for a replacement water chip for Vault 13, it was already ransacked and had also undergone complete power failure, several cave-ins, water damage, and other catastrophes. The intervening years has not done anything to improve the vault, but they have brought new occupants. When the Vault Dweller massacred the Khans in 2161 a sole survivor Darion was corrupt with guilt because he alone had survived when the rest of the tribe died. Darion then spent the next 8 decades of his life rebuilding his petty tribe to the once great gang that scoured the wastes, also out of revenge against Tandi and the Vault Dweller. The result was the New Khans. A small settlement of squatters has made its home at the entrance to Vault 15. The squatters forge a precarious existence there by the sufferance of the gang.
Shortly after, the squatters allowed Darion and the rest of the Khans to move into the vault itself on the condition that they repair the vault's systems in order to help provide the squatters with food, water, and other amenities. At first, this setup was advantageous enough that the squatters rejected all efforts by the NCR to annex them or even gain simple access to the vault.
Then, the Chosen One came along. President Tandi hired the Chosen One to infiltrate Vault 15 - or at least get the squatters to allow NCR access - to retrieve computer parts and other equipment vital to the NCR's continued expansion. While accomplishing this quest, he discovered the truth behind Darion's operation; he and his men were actually a band of raiders calling themselves the New Khans, using the vault as a base of operations towards Darion's goal of revenge against Tandi and everything the Vault Dweller stood for. Also, the food and water that he had been providing the squatters with didn't come from the vault's systems after all, but the spoils from raids the New Khans had been running along the wastes between NCR and Vault City.
After Darion was killed by the descendant of the man he had sworn vengeance against nearly eighty years earlier, that very same man brokered a deal with the squatters, who accepted the NCR's assistance. With the support the Chosen One secured from the New California Republic, the Vault 15 squatters soon became self-sufficient and productive members of society. The Chosen One's help with Vault 15 launched the New California Republic's push to civilize its neighbors. Though there were many more obstacles to overcome, the NCR now had a foothold into the northern wastes.
Due east from Vault 13, construction on this Vault has gone extremely smoothly. Much work was done to reinforce the walls of the third level of this Vault, to make all the future Vault Dwellers more secure in the knowledge that in the event of even a major earthquake, the regulatory computers of the Vault would continue to function. Recent tours of the newly constructed Vault, have had many a potential Vault Dweller walking away with a new found awe of the improvements done to this already impressive Vault.
This area has a layout much like Vault 13. Here most people will get their first armor. You will need a rope to go down further at the end of this area. In the 3rd room counting from the left you can find a locker with a rope in it.
Much of this level is buried under tons of rock and rubble, including the command and control center. The rest of the vault's equipment has been ruined or destroyed. There are many pig rats, rats and one mole rat.
Vault 15 can be found nine squares east of Vault 13.
Areas of Vault 15 which were blocked off by fallen rocks in Fallout are accessible in Fallout 2.
If you examine the Fallout eastern U.S.A. map carefully, you might recognize that Vault 15 is close enough to the Mojave Wasteland.
In the entry tunnel in Fallout 2, there is a ladder that leads straight to the exit grid in "The Squat". Through The Squat however, there is no way to enter the tunnel, until The Squat has joined the NCR. This ladder becomes accessible through Zeke's house.