I'd say that even in the two games that he has been back, Clark has been the better of the two.
And unfortunately that **** up did happen, so it kind of ruins a lot of the credit he would have otherwise got from yesterday's game. Clark played just as well, if not better, and also didn't give away an injury time penalty (therefore losing us the game).
Centre backs play as a pair, so you have to consider the effect that one has on the other. Clark has played well alongside Vlaar, but he was often taking up a defensive position far too deep when playing alongside Okore.
I would say that Vlaar improves Clark, and therefore the whole defence, so his contribution isn't just what you actually see him doing.