Sure, but Vettel already won in China and RB was on the podium pretty much all the time after that. If I recall it correctly, it took them a few more races until they introduced the double diffuser. The RB5 was a good car even without the DD and the RB6-8 have all really only been evolutions of it.But you see, the RB5 did have a double diffuser.
When it did not in the beginning of the year it was beaten or dominated by the Brawn GP cars,
It was after the put the DD on it that it really started becoming competitive that year.
Edit: Okay, I did not recall it correctly. They already introduced it at Monaco. Still the RB5 was leaps and bounds better than the RB4 and the China win was still achieved without it.
And as you said yourself, the dominance of the RBs over time with the current set of rules has gone down. The RB6 was the most dominant, then the RB7 and the RB8 only really came alive with Valencia and then the updates after Monza. The longer the rules stay the way they are, the weaker (or rather: less dominant) RB becomes.