He finishes in the top four
He finishes 5th, 6th or 7th and Alonso is 2nd or lower
He finishes 8th or 9th and Alonso is 3rd or lower
He finishes 10th or lower (including not completing the race)
if Alonso is off the podium

Alonso wins the title if:

He wins the race and Vettel is 5th or lower
He finishes 2nd and Vettel is 8th or lower
He finishes 3rd and Vettel is 10th or lower

Tied on points after the race?
The crown goes to Vettel on the basis of more race wins.

Who will win the 2012 Formula One world title?