This sounds a lot like an idea I had eight years ago for a Fallout prequel that I dubbed Fallout Zero. The beginning of the game takes place just before the War breaks out. You play an American lieutenant in charge of a rapid response platoon in occupied Vancouver. You get to experience life on the American base, and the activities you participate in help determine your SPECIAL stats and tagged skills. You also get to meet other characters who will be important later. On the morning of October 23, 2077, your unit gets sent out to Vault 31, outside of the urban area (Stanley Park might be a good location for it). A band of Canadian insurgents has broken into the Vault in order to steal supplies. On your way there, your vehicle is disabled by an IED and your platoon is ambushed by Canadian insurgents (possibly connected to the ones who are raiding the Vault). This gives you an initial taste of combat, though there's also the possibility of repairing the vehicle while under fire and fleeing. Assuming you aren't killed, you and any other survivors of the ambush then proceed to the Vault. Alarms are going off, indicating that the Vault has been breached. You make your way through it, during which time the War breaks out. However, the alarms are already ringing and the Canadians have a jamming device to block you from communicating with the outside world, so you don't notice anything different yet. Just as you reach the bottom of the Vault and confront the Canadians, the bombs fall and the Vault automatically seals itself. You can at this point choose to either fight it out with the Canadians or try to make peace with them. The decision you make here, as well as which members of your group survive, has a large impact on the rest of the game.
Fast forward seventeen years. Radiation levels have finally died down enough that it's relatively safe to venture outside. Supplies are running low (this Vault was intentionally understocked as part of the Vault experiment), and you need to explore the outside world to find enough to keep your people alive. There are other survivors as well, including some from the military base, which had its own built-in shelter. Your former commanding officer has gone completely mad and set himself up as the head of a powerful pseudo-religious cult, along the lines of Apocalypse Now. One of your fellow officers who was your friend is now his right-hand woman - and lover. The cult dominates the region, but there is also much more to explore along the Pacific Coast, possibly getting as far south as California.
