I don't really think it make sense. Wasn't USA the only place with the Vaults, wouldn't the rest of the world just be Ghouls? I mean there probably were some Fallout Shelters but they weren't designed to keep humanity alive like the Vaults.
Im 90% sure fo3 talks about vault tec vaults being in Canada. You can find info like this by "mindlessly exploring" mailboxs.
If my memory serves the letters from vault tec to potential customers tell them the DC vaults are full, but they would be put on a waiting list for some vaults in middle America or newly annexed Canada.
No telling what other governments did to ensure that everyone would not die. I m pretty sure everyone will not die anyway.
The people that built Megaton never made it to a vault
Rich people build there own shelters. Governments would tell people where the safest places to go would be.
Most scientists don t think everyone will die.