Once in the vault, you, your family and other future vault-dwellers are put into stasis for the duration of the war and would be let out of their stasis unit then to take full control of the Vault until such time when heading back out becomes viable. But of course that is what everyone is told but as we know about Vault-Tec, each Vault is actually a (laughably) controlled experiment. Periodic equipment malfunction causes certain number of stasis units to react in various ways. Some thaw out its occupants, forcing them back to reality a lot sooner than expected (one of them being the game's antagonist the "Overseer" [predictable but in a Vault the options are quite limited.]), some are thawed out but stuck in the stasis units until they die, thawed out with explosive results or are burned alive or somehow their air is tainted by some form of the FEV virus, causing them to mutate and even some are dosed with various levels of radiation leading to ghoulism. Yadda yadda yaaa, you get the idea.
Since the Vault would be a true underground enclosed habitat, it would be huge and I don't mean the pathetic excuse of what is considered a Vault in the Fallout franchise. I mean a true underground city with its own mass transit system and whatever features a normal city would have.
Because of the various types of beings now reside inside the Vault, sections of the city are sectioned off by groups. Humans have control of about 40% of the Vault including just about every strategically valuable areas like the armories, clinics, overseers office and so forth. Non ghoul mutants have control of 30% of the Vault, ghouls 10% and the rest is a no man's land which separates humans from everyone else.
After the intro, it is two hundred years later and you see a small armed group of vault-dwellers searching through the lowest level of the NML's area of the Vault for anything of value that would help them "purify" the Vault but instead stumble upon some working stasis units. Confused as to why these units are located in the most remote part of the Vault, away from all the other stasis units, they decide to thaw out those inside only to find everyone (including your family) but yourself had been killed off in various ways but not before being experimented on. You come out of stasis but fall to the floor before taking your first step but one of the vault-dwellers help you up and sarcastically welcomes you to "Vault City". But before you can answer, growls, snarls, howling and inhuman laughter is heard echoing from the hallways and you are thrown over the shoulders by one of the team's burliest members and are hustled to safety. The last thing you remember before blacking out is the screams of your rescuers and the sounds of weapons' fire.
A short time later you wake up in one of the clinics and you are greeted by a nurse who then calls for your doctor. She appears and gives you the standard Fallout facial make-over and beginning stats. You are then given a quick rundown of what had occurred since the Vault was sealed until now but she then tells you that more details can be found in the city's library.
Flash forward a few years and you are now sitting in the guidance counselor's office where he\she tells you that you have great potential in whatever field you wish to study but you need to take the G.O.A.T. (insert whatever reason). You take the test and you are flash forwarded to the present time in which your character is in whatever field you chose (Doctor, soldier, politician, criminal, etc...). The first few missions are tailored to the profession before you are pushed to the forefront of a special mission that if successful would ensure a victory for humanity. Along the way you learn about what really happened in the vault and by the end of the campaign against the non-humans, you learn that the Overseer was behind it all. You also begin to wonder how the Overseer managed to stay alive throughout all this time and you get the answer from the dying breath of the leader of either the ghoul faction or the mutant faction. From there your real mission is to get out of the Vault alive, even if it meant being everyone's enemy. The only escape is through the Overseer's office, forcing you to confront him in a climactic battle. You live, everyone finds out what the Overseer really was and a mass exodus begins with you leading the way.
Of course as in every Fallout game, there would be multiple endings.
This is kind of slap dash but it can be fun.
