Vettel has said in the past that sometimes the team will let him go for the fastest lap so that he can get 1. pole, 2. win, 3. fastest lap, all in a weekend. That also implies that they don't let him do it all the time though.
So just because he doesn't get it doesn't mean he isn't the fastest on a weekend.
I'd like to believe that mclaren can challenge for the win but just because they are setting times quicker than the cars in front, doesn't mean they are quicker.
That said, Spa doesn't seem to need as much downforce either. Remember when Fisichella put the force india on the front row 2 years ago?