i don't believe in the conspiracy theories. however, i do believe that rbr was forced to split their strategies because of the lotus, and therefore they had to put one driver on a two stop, and the other on a three stop.
given that, it makes more sense to put webber on the 3 stop as he's historically not as easy on the tyres. so vettel then gets the 2 stop, which is also accepted as the superior strategy, and so you should give it to your best driver.
had they not been forced to split strategies, they would have kept webber on the 2 stop i believe.
once they made the split, they knew that vettel was the likely winner. hence the radio message telling him he's racing grosjean. yes, he knew he was on the same strategy as GRO, but he didn't know what the situation would be at the end of the race, hence the message.
as for webber being unable to pass, he was on used mediums (i believe?), but also given his lower downforce, he was unable to get close enough to the lotus at the start of the straight after the chicanes, and hence even with his higher top speed was unable to pass. vettel on the other hand was right on grosjean's ass at the start of the straight and was hence able to get past. interesting, as i thought webber's lower df setup should have made life easier for him, but perhaps not.