Vettel has won the title, this isn't the last race of the season, mathematically others can still win, in reality they won't, it's already won. RBR know that, there is entirely no reason to take a last win from Webber to secure it one race earlier when it was effectively won a couple races ago. What is it 77 point gap, 125 available, I would put money on Alonso not passing Vettel's current total.
Checking skybet, Vettel 1/500 to win, Alonso 66/1 to win, and that is probably being generous.
I think Vettel wouldn't pass Webber here even if he could unless Webber has an issue and Vettel would be losing places or loads of time to stay behind him. Vettel screwed him earlier this year when the title was up for grabs, it's won now but more importantly he knows the reaction the fans would have and the positive reaction giving Webber a win would have in terms of how fans regard him.