The vaults built by Vault-Tec in the Fallout franchise were originally meant as tests for space exploration. In the world of Fallout, Vault-Tec vaults are used to protect people from the nuclear apocalypse. People can practically live in the vaults indefinitely, though the Fallout video games tend to revolve around people who leave their assigned vaults to explore the apocalyptic wasteland for whatever reason.
For example, Fallout 3 is the story of a person who was born and raised in a Vault-Tec vault, but they eventually leave to search for their missing father. While out exploring the wasteland, Fallout 3's main character, aka The Lone Wanderer, can find a variety of other vaults that Vault-Tec built in the area. The stories of these vaults differ, though most of them ended in tragedy and ultimately failed to keep their occupants safe.
As explained in a recent YouTube video by Fallout creator Tim Cain, the Vault-Tec vaults were meant to be a way to solve various problems with space travel. The vaults could be used to test various conditions that would be relevant to those traveling through space, as well as figure out how to do things like grow plants and have a sustainable water source.
What's interesting about the Fallout vaults being meant to run tests for eventual space travel is that Bethesda's next game is all about space travel. Bethesda took over the Fallout franchise starting with Fallout 3, and Starfield effectively looks like a Fallout game set in space. It retains many of the gameplay features that Bethesda fans expect from the company's game, but with thousands of planets to explore.
Starfield should go a long way in scratching the Fallout itch for fans of the franchise, but others may be more interested in a new Fallout game. There have been rumors that a Fallout: New Vegas sequel may be in the works, but nothing official to that effect has been announced at the time of this writing. But while a Fallout: New Vegas sequel may or may not ever happen, Fallout fans do know that they at least have Fallout 5 to look forward to at some point down the line.
When Bethesda finishes Starfield, it will turn its full attention to The Elder Scrolls 6. Bethesda's Todd Howard has already confirmed that Fallout 5 will be the studio's next game after Elder Scrolls 6, but fans likely won't learn more about it for years.
Source: IGN