If there's one thing Bethesda does really well with the Fallout series, its world building.
Pretty much everywhere you go there are stories to uncover which might span a huge epic quest or be as simple as a body clutching something in its hand. Many of these stories end badly (we are in the post-apocalyptic Wastelands, after all) but when you look into the history of Fallout's iconic Vaults, they have some of the most disturbing stories in the entire franchise.
As of writing, there have been 29 confirmed Vaults which have appeared in some form in the Fallout universe. Several others have been alluded to, but there's too little information on them yet to formulate what happened there.
You'll be utterly amazed at what the intended purposes were for these Vaults. As you get further down the list, the stories get darker and darker, and it's amazing to think you might miss some of these incredible stories if you didn't read all the logs available in the Vaults.
So, prepare yourself. We'll ease you in nicely with some nice gentle Vault action, but be prepared for things to take a turn for the worse very rapidly as we descend into the screwed up world of Vault-Tec and their Vault experiments.
29. Vault 88 - Never Finished
Added in to Fallout 4 as DLC, Vault 88 is easily the least screwed up out of all the Vaults.
There's one reason for that: it was never finished! Instead this Vault only has a main entrance, security and reactor rooms.
The entire purpose of this in gameplay is its designed to be used by the player as a Settlement in Fallout 4, and they can customise it as they see fit.
God knows what the Vault would have been used for if it was actually finished.