Regardless of the dynamics between Vettel and Riccardo, non-Merc teams are going to have a near impossible task in bridging the gap in engine efficiency and max power. The design to which Mercedes committed itself is a brilliant interpretation of the rules that other manufacturers likely cannot adapt to mid-season. Its a question of basic architecture that Merc got more right than Renault and Ferrari.
If the estimates of 12 mph difference in straight line speed with similar drag are accurate, its going to be a long season for anyone not running the Merc power plant. The Mercedes works team will make improvements to mechanical grip and aero at about the same pace as everyone but possibly Red Bull, who have the A. Newey card. I just don't think that's enough to beat Mercedes, given the advantage that will likely exist for more than half the season.
Here's a pretty good explanation of Mercedes genius:
Formula 1® - The Official F1® Website