Red Bull seems to be uncomfortable with the set up. Vettel's car has been messing with the exhaust configuration, set up, parts etc...
they even changing the torsion bar and tweaking set up the entire weekend. Vettel seems have not found the confident and comfort zone with the RB8.
And Anthony Davidson pointed out a few tell tale signs of Vettel's mistakes, where he had to miss the apex to avoid spin out because the rear locked up. And fighting with the rear end grip.