The reason that The Vision didn't manage to stop Steve Rogers/Bucky Barnes from getting away is two-fold.
When they are first escaping, he is trying to make sure that Wanda Maximoff is OK. Earlier on, Rhodey used some kind of sound-based attack on her that she had not yet fully recovered from. At this point, his attention lies with Wanda.
However, Tony soon asks him to "turn that wingsuit into a glider" (or something to that effect) - i.e. shoot Sam Wilson's wingsuit so that he has to stop attacking Tony and Rhodey, who are in pursuit of Steve and Bucky at the time. The Vision messes up his shot, and shoots Rhodey out of the sky - at this point, all attention turns to trying to save War Machine as he plummets towards the ground.
In short, the reason that The Vision doesn't continue pursuing Steve and Bucky is that he is, in his own words, distracted - first by Wanda's condition, and later by the fact that he accidentally put his ally's life in danger.