An active vision system has to enable the implementation of reactive visual processes in real time. Given a stereoscopic vision system, the vergence angle, together with version and tilt angles, describes uniquely the fixation point in space. We interpret vision and motor control, and in particular we focus on developing and testing of a control strategy that fits the Hering?s law, by studying the cooperation of vergence and version movements. The analysis of the simulation results confirmed the advantages of the Hering?s law to achieve fast system reactions. We show that real-time active vergence and depth estimation become possible when the estimated disparity is reliable and fast. In this framework, the advantage of a simple and fast phase-based technique for depth estimation that allows real-time stereo processing with sub-pixel resolution is also discussed.