This paper presents a novel approach for vision-based control of the end-effector of parallel mechanisms. It is based on the metrological redundancy paradigm, which simplifies their kinematic models by introducing additional proprioceptive sensors. By observing the mechanism legs, vision replaces advantageously these sensors by delivering, in a Cartesian frame, an exteroceptive measurement of the internal state of the mechanism. Formally, the latter is expressed by an original concept of vision-based kinematics for parallel mechanisms. Based on it, control is derived that visually servoes the direction of the legs, rather than the end-effector pose. The method is illustrated and validated on a Gough-Stewart platform simulation.