|The following is based on Van Buren and has not been confirmed by canon sources.|
Discover the vault was going to be a quest in Van Buren, the canceled Fallout 3 by Black Isle Studios.
There is a vault hidden in the forbidden lands. The Prisoner must find the entry and discover a way in. This can be accomplished by breaking and entering, disguising as a native, or blowing shit up.
Twin Mothers design document