Webber moved left to the far extremities of the circuit. Hardly enough room for Vettel to come through. As Vettel came alongside, the natural reaction of Webber would to start moving over to the racing line (Far right) to help him negiotiate the corner better than Vettel to help an make a better exit and re-pass.

Vettel, Naturally during a move like he did would start to make his way over also, usually with the guy next to you your passing.