Thread:ImtheSteve/@comment-24416658-20150930063615/@comment-5490216-20151213193639

He wrote that they were sorta like expansions for the Vault program cause it was popular or whatever (I mean if you have an apocalypse and you paid on credit, kinda doesn't matter anymore, does it?) And I think somewhere in the canon, it said there could have been more vaults that weren't in the initial Project Safehouse.

Hence Fallout Shelter having Vaults up to 999 :P