Vick also throws many "panic" quick throws that are very difficult to catch. His timing is all off and his recievers really never know when he will release. It almost looks like a pickup game out there when they are on offense. Sometimes he runs, sometimes he doesn't. While this can give defenses problems, it also gives their wr's problems.
He's just not smooth out there and the result is wr's dropping balls. Brees has timing and throws nice "cupcake" balls(that are deadly accurate) that are not only easier to catch, but easier to run with after the catch.
Yes, the Falcon wr's dropped some easy balls, but when you have no idea where, when or if the ball is going when its coming to you, its a lot harder to make the catch. Seems to me they are constantly having to make adjustments to the ball when it is coming at them. It also seems to me that the wr are confused where to run once Vick starts moving around a little or a running lane opens up for him.