In all fairness, he only caught 55 games during Cliff Lee's Cy Young season. But you are absolutely right, when guys like CC, Lee, and Lester are on, and locating their pitches, it really doesn't matter who is catching.
In all honesty, I wouldn't be surprised if VMart costs .5 to 1 runs a game for a guy with slow pitches like Wakefield, but its not VMarts slow reaction time that's costing so many runs at this point, its the starting rotation sucking, and until the rotation warms up, there is no way we can know how relevent he is to runs scored.