You can return inside the vault in Fallout 1, I remember doing that soon I started. And the building in Fallout 2 beginning wasn't a vault, but some type of temple where they stored the Vault 13 jumpsuitIn Fallout 1 you start outside the vault but you are coming from inside the vault and cannot immediately re-enter. In Fallout 2 you start just outside the vault and your only path is to go inside the vault. In Fallout 3 you grow up inside the vault until you are forced to leave. I always saw Fallout: NV as a spin off so I think it is likely to have a vault related beginning in F4. Will be interesting to see how they can keep it fresh though.