I think that Vettel might yield for Alonso in the last laps of the race. I believe that Seb will deliberately fall back in the last minutes of the Brazilian Grand Prix to make sure that Alonso wins the title.
Vettel knows that he’s still very young and that he’ll have many opportunities to win more titles. Since everyone talks about Newey and how Seb doesn’t really deserve the championship, he probably doesn’t even want it anymore.
What is more, if Vettel really is about to join Ferrari in 2014, then his heart is already in Maranello. I’m sure that Ferrari will know to appreciate Seb’s humility as well as his readiness to work for the team and will reward him appropriately.
EDIT: All of this is, of course, nonsense and I fully agree with @atticus-2 .