I have the same problem with one of my vaults.. I'm currently busy trying to navigate the savegame's data, while my game plays off a fairly recent backup (only 1 day so not much lost)...
I think this is because quitting the game via main menu and then swiping left or right the program in the list so it's definitely closed... Somehow this corrupted my last savegame and I'm working on a way to restore it but so far no luck.. If you happen to know a programmer for c, c++ or Java ( lots of {} things and text in between) let me know. Has to be an ethical person though because of what else is in the savegame's file....