How Vettel can win the world championship in Singapore (Source: The Guardian)
Vettel has to be on the podium at the Marina Bay Street Circuit to have a chance of taking the title, and only once this season has he not finished in the top three. If he is fourth or lower, the title race moves on to Japan, no matter what happens to any of his four rivals. At best, with 12 points for fourth, he would be 124 points ahead (with 125 points available).
If Vettel wins
To take the title Alonso then has to finish fourth or lower, with Button and Mark Webber third or lower.
If Vettel is second
Alonso has to finish eighth or lower, with Button and Webber fifth or lower. Lewis Hamilton must not win.
If Vettel is third
Alonso has to finish ninth or lower, Button and Webber seventh or lower. Hamilton must not finish in the top two.