Vettel passed on the exit of a corner tbf, but gave little room to the HRT and in the wet, where aquaplaning is common, it is asking for trouble. Coulthard actually called it right (for once) imo, a racing incident.
I thought at first Vettel had moved as much as Karthikeyan, but actually the track curves to the left (as the driver sees it) so it was deceptive.
Button was gentleman enough to admit he made a mistake against the HRT, locking up his brakes and sliding into Karthikeyan, although I think he was unlucky to lose his wing.