Vault 25

After considering your options, you ultimately decide on making up your own Vault rather than choosing a preexisting one to take over. Giving a quick skim of the canon Vault list to double-check which numbers are already 'taken', you settle on the unclaimed 25 for your Vault.

Now for the big question: what is the purpose of Vault 25? Obviously the real purpose of the Vault is to allow you to live out a Fallout-themed sex fantasy, but in-universe the Vault was most likely built to conduct an experiment on its residents. So what kind of experiment was that going to be? After a little bit of thought you come up with one that you think will be rather fun.

It's a rather simple 'experiment' as well, enough so that it could be summed up in a single sentence, which you make sure to write down: 'The Overseer of Vault 25 will be the ultimate authority over any and all sexual activity of its residents, and may create and enforce any rules pertaining to such.'

There. Vault-Tec probably would see it as an experiment in whether one person given power over the sexual activities of an entire community would prioritize repopulation and survival over satisfying their base urges, but your intent is to make sure that the Overseer of Vault 25, that is you, can basically do whatever they want as long as it has a sexual justification, and can even dictate how everyone else gets off. Perhaps you'll make it so that everyone must orgasm at a certain time each day regardless of where they are, or that all women need to wear butt-plugs outside of their dormitories, or even restrict who gets to have sex with who. You could even make rules and restrictions that only pertain to certain individuals, if they ended up displeasing you. In a typical Fallout story, such draconian rules would usually end with a Vault-wide revolt, but you could easily use your powers to ensure that Vault Security is completely loyal to you and will help enforce any rules you make, while being highly efficient at thwarting any serious attempts at resistance from the rest of the population. Assuming you don't decide to completely brainwash the residents into being 100% happy with this outcome, it may be interesting to see how they deal with such conditions in the long term. How the average dweller will react to you and security will likely depend on whether you decide to insert yourself shortly after the Vault door seals shut, or later on once people have generally come to accept the new society that they are trapped in. Hell, if you come a generation or two later, you may even decide that you don't particularly like how your predecessor ran things and that changes need to be made, which will surely shake things up for the population of the Vault who may have grown complacent towards how things were done before you took over.

The possibilities are limited only by your imagination and willingness to carry them out. Just remember, the Vaults were never meant to save anyone, so don't feel too bad if not everyone in Vault 25 ends up happy with how things turn out. After all, there are far worse Vaults they could have ended up in.