Hello everyone,
Thanks to suggestion of ElderLyons10 I decided to add in every vault my own story in order to bring more background as well as inspire GMs for their own games. Here you go:
Due to cruel experiments, the inhabitants of Vault 10 became morbidly obese. The only upside to their condition was that they required very little, if any, water to survive. However, their immense size rendered them almost immobile, and when the notorious Raider Band attacked the vault, the dwellers could do little to defend themselves. Unable to wield conventional weapons because of their swollen fingers, and incapable of running or hiding, they were massacred by the raiders. Cannibalism was common among the raiders, and after slaughtering the dwellers, they even resorted to eating them.
Despite the chaos, the raiders were unable to breach the innermost part of the vault, where the Seer’s office and correction facilities were located. Thanks to a backup fusion generator, the G.E.C.K. room remained untouched. The Seer, realizing the hopelessness of the situation after hearing the slaughter outside, chose to take his own life, sealing the G.E.C.K. room permanently with his password.
Unaware of the full story, the players join forces with the NCR to defeat the Raider Band, eventually succeeding. Once the NCR takes control of Vault 10, they gain access to the inner sections and discover the unused G.E.C.K. Using it, they cleanse the surrounding area and establish a new settlement called Vault Town. The transformation took six months, but the results were extraordinary. When the players return, they find a thriving town with some of the most beautiful trees they've ever seen in the wasteland. As a reward for their help in building the town, the players are granted a lifetime entry permit. The small BoS bunker outside the vault, which was found after clearing the raider base, remained sealed since none of the NCR experts could find a way to open it. Until one day, a very talented hacker (player) managed to open the door...