While many people talk of Monza 2008 as proof that Vettel can perform in a bad car - which I agree with - I think as JET says, the true test will be how will he perform in a bad car, AFTER he's had a championship-winning car.
Monza 2008 was proof that he has the ability to do it in a bad car. But at that point he was still trying to prove a point and get a seat at a top team. But if he's had a very good car and has to move to a midfield-ish car; that might be a big question mark.
As an analogy - it's very easy for people to move from a 13" laptop to a 15" laptop - but quite hard for a 17" laptop user to move down to 15" in terms of screen real estate