What happened was that Vettel caught Karthikeyan just as they went into a section of track where it was very difficult to go off line to allow someone to pass. Karthikeyan had spoken to Charlie Whiting before the race as, driving an HRT, he was concerend about how to handle that situation. Whiting had told him that it would be OK to stick to his line in that section then allow the cars to pass after, which is what he did. Unfortunately for Vettel that allowed Hamilton to close up enough to get past, and Vettel whined to the team about it.