You're a courier, wounded and left for dead in a shallow grave. A friendly robot, Victor, digs you out, and his doctor owner Mitchell patches you up. You take a "vigour test", which is some sort of electric parlour game. This decides who you are and sets up SPECIAL. You can also take some Rorschach tests, but the mag says this is for fun. The Doc then gives you a Pipboy as he was once a Vault dweller.
Given that it's Caeser's versus Vegas Citizens versus NCR, I'm hoping the doctor has a bias towards one (probably the NCR, them and vault dwellers probably have the most in common) and saved you with the intent of making you an NCR agent. Then, GASP, you find out the NCR are as ruthless and corrupt as everyone else and the man who saved your life only did so because he knew you'd owe him.
I'm also hoping the BOS isn't ingame at all. I'm sick of them.