Okay, so there is a cellar door in this demolished house in Springvale that is on the right as you exit vault 101. Not the Springvale School. A Springvale house. The problem is it requires a key to open. Does anyone have any idea where the key is?

That door is opened once broken steel starts.

It becomes the home of am offshoot of the cult of atom.

Oh, ok thanks. Guess I should play broken steel so I can check it out.

Huh, so that's what it's like pre-BS. Interesting

That it is used in BS is kind of odd. I find it difficult to believe that they put that cellar door in with the intention of using it later. I had wondered what it was as well and I used the console to unlock it in the vanilla game, but it went nowhere and did nothing. My guess is that it originally had some function in the vanilla game, but where ever it led to got scrapped and the door was never removed. So when BS came out, the Devs took advantage to tie up a loose end. But that is just my own personal opinion.