Although this starts in Vault 73, the world rules extend beyond the vault door. If you leave you can keep playing outside. You can see your health & stuff at any time by typing /status.
You were traveling through the Colorado wasteland when you were ambushed. The HalluciGen Gas Canister rendered you unconscious within seconds.
You wake up in Vault 73. You’ve never been in a vault before but you’ve heard stories. The room you’re in looks medical in nature, but unlike any you’ve seen before; it’s actually sterile. Besides a Mr. Handy floating around the room, you are alone, lying naked on an examination table.
Mr. Handy: Oh, how convenient. You're awake!