Vault 91

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 91 for your Vault.

Now for the big question: what is the purpose of Vault 91? 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.

Upon entering Vault 91, all of the Vault Dwellers would be put on a mandatory daily **** regiment, to be administered by a small group of Mister Orderlies that also act as helpers to any human medical staff present. The residents will be told that the **** they are taking are to help protect them against potential radiation poisoning, but the true purpose of the **** is far more sinister. In fact, not all residents will be getting the same chemical cocktail to begin with.

The women of Vault 91 will be given an experimental growth treatment which will make them grow taller and stronger, maxing out at a bit over seven feet tall on average. Of course, this won't make them lose any of their femininity: in fact, in some test subjects those same growth hormones may even increase the size of their hips, butt, and bust on top of everything else. However, these **** also have the side effect of damaging certain areas of the brain dealing with critical thinking and memory retention, effectively making the female population of the Vault become dumber as they continue to grow.

The men of Vault 91 will be given a series of **** that will do pretty much the exact opposite. Their memory retention and critical thinking skills will increase during their treatment, increasing intelligence for them across the board, but these same **** will also stunt their growth and make it harder to build and maintain muscle mass, eventually making the male population smarter but physically smaller and weaker than their female counterparts, with a final projected height of around four feet.

The purpose of this experiment, at least in the eyes of Vault-Tec, would be twofold. The first would be to test the effectiveness of these **** in general, and see if the potential benefits of each may outweigh their downsides. The second would be a sociological experiment to see which of the two genders would take control of the Vault once the **** treatments reach their final outcome. Will the stronger but dumber women use their physical strength to intimidate and strong-arm the men into serving their desires? Or will the weaker but smarter men convince the women of the Vault to let them do all of the thinking, and that it is in their best interest to follow any orders given to them by a man. Perhaps the two groups may even manage to find a balance that lets them maintain a surprisingly high level of gender equality despite the new size and intelligence gap between the two of them. In essence, Vault-Tec wants to figure out whether brains or brawn would win out if they were both pushed to an unnatural ****.

Of course, you'll be able to use your powers to rig the results towards whatever outcome you desire, or even one Vault-Tec never could have imagined. Perhaps you could add aphrodisiacs to one or both of the **** regiments, making the subjects more focused on using their new gifts to satisfy their urges, or you could add other side effects to the **** to encourage specific sexual behaviors and influence what power dynamics will eventually emerge. You can also choose how fast the ****-fueled transformations take place, having it take place over weeks, months, or even years. Maybe the regiment doesn't have any effect on adults at all, and thus only the future generations of children that grow up in the Vault would be changed, creating a generational divide as well as a gender one. And you might decide to make it so that you personally somehow managed to not suffer the negative side-effects of the ****, making you a hunk among men or (if you wanted to change your gender) a genius among women. Hell, you could even go a step further and give yourself the best parts of both treatments if you really wanted to ensure your superiority, or make it so that you were somehow exempt from the **** trial altogether in order to be the only normal person left in the Vault.

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 91 ends up happy with how things turn out. After all, there are far worse Vaults they could have ended up in.