I came up with this challenge while playing around working on Pie_Snipers
Two-Door Challenge. His is more difficult, because of a special caveat, which I will detail below.
I found it is indeed possible to get the Vault Ending by unlocking only one single locked door. (Other doors may be used as normal.)
No cheat or developer mode.
-The two door challenge restricts the player from entering the Village. This challenge does not!