RB was one of the only spots without competition going into camp. They wanted to give the guys projected to make the roster better competition than Terrell Jennings as a camp body. Vrabel has said many times (in a more eloquent way) that he wants his players to have to fight for their spot. To be pushed. He really believes in it.
My guess is he saw who they were up against and that Gibson and Larison were too comfortable. Wanted to give them more of a challenge. Looking at the roster now, i don't think he looking to add competition anywhere else. Some spots will have more, some will have less, just how it goes. But i think he feels he has reach the minimum amount needed to light a fire under anyone not considered a lock already. If he were to add someone else he may go TE. But I'm guessing he thinks Bell is enough to push Dipree.