Barrichello has the experience and consistency to stick to 2nd. Vettel is still testing his own limits, and will definitely make more mistakes.
Brawn and Button like to understate their car's performance, always saying something like how the rest of the field isn't so far behind, how Button isn't happy, etc. It's a BS ploy for their future sponsors. The Brawn is comfortably superior to any other car on the grid, and every driver and team knows it. Only wet races give it any trouble, but even in the wet, it's still faster than all but the RBR.