(Possibly 1% of this is cannon. Vault-Tec only created 122 vaults) Vault 122 was the last vault created and was unfinished due to the great war. The vault was meant to be stashed with riches and filled with greedy people just to see what would happen. Luckily, the jewels and gold bars were stashed before construction came to a halt. This vault is located on the outskirts of Chicago.
Vault 122 was significantly close to Chicago, meaning it was close to radiation and heat. When the alarms went off, there were Vault-Tec employees and construction workers at the site. Being the stupid humans they were, they went inside the unfinished vault and closed the non reinforced door behind them. During construction, a false door was placed inside to test the door opening and closing mechanics. They rushed to the very back of the vault with all the riches. Once the bomb detonated, the air pressure forced the door into the vault which let radiation and heat in. The vault, not only for testing greedy people, had their air conditioner set to far below 0 degrees Fahrenheit at random times. The people inside set it to that very condition, but their attempts at survival failed. Not only did the heat still kill them, radiation would've killed them anyway. They basically killed themselves and thousands of other people by not finishing the vault in time. The iron door lay across the floor in the room with the door and a few dozen skeletons in the unfinished treasure room. Defenses, though, were very good and could kill tens of deathclaws that tried to get in. The defenses could take down 2 behemoths.
Construction started in 2072 and ended in 2077. The vault was unfinished and will remain that way forever due to lack of resources, care, and knowledge to do so. The vault was the last vault and wasn't given much enthusiasm which kept it from being finished.