Since the original thread is lost somewhere, I'll just start anew one.

My idea would be a vault that wasn't part of project safehouse at all. In fact it was an unfinished (done by privates) vault, that relied on some of Vault-Tec's technology such as the vault door. It's mechanism was already intact and so it closed as the nuclear warning was given out Oct 2077.

Now many wastelanders in the coming decades knew about the vaults of course. Others had already opened, GECKs were found and used and as one such device (and other pre-war treasures) were suspected in the aforementioned vault (not having a number was unusual, but not of further interest), communities established in it's proximity. They might have even fought for the right to take it's bounties. The vault would remain closed for a long time, until it finally opened and revealed: Nothing at all.