Bethesda have dropped a teaser trailer for Fallout 76, which according to Kotaku’s Jason Schreier “is an online game of some sort. Developed by BGS in both Maryland and Austin (formerly Battlecry).” An online Fallout does sound wonderful, hopefully it will be closer to Fallout 4 rather than the cancelled hero shooter Battlecry.
The music seems to suggest the game may be set in Virginia, and that the Vault 76 occupants were meant to be America’s best hope after the nuclear fallout. The vault seems to be functioning well although the lack of other vault dwellers at the birthday party is rather ominous.
The Fallout Wiki has this to say about Vault 76.
Vault 76 is one of the Vault-Tec vaults shown on the computer in the Citadel.
The Vault-Tec terminal in the Citadel lists Vault 76 as a “control” vault, with 500 occupants. The vault was designed to open 20 years after a nuclear war, and was among the seventeen known control vaults, meaning that it was used as a baseline to compare to experimental vaults. It is also mentioned in one of the alien captive recorded logs (no.13) recorded by Giles Wolstencroft, a Vault-Tec official who was abducted while inspecting the vault’s construction site. In Fallout 4, the newsreader in the prologue mentions Vault 76 debuting in 2076 in honor of America’s tercentenary when discussing Vault-Tec’s plans to expand.