The Red Bull is not very good this year, simple as that. They lack straightline speed, they lack qualifying pace, their aero is compromised because of the new rules and still they win three races and so far lead the WCC. It won't last, but is still a mighty effort.
Vettel had competitive race pace in Spa. Arguably the quicker car and perhaps would have won the race had he had qualified higher up the grid. I don't fully agree that RBR lacks mechanical grip. Equal in race pace in Australia; they won in Monaco and perhaps would have won the Canadian GP if they had gone for 2 stopper.