When using vsync, mouse movement is less sensitive than when vsync (and frame limiting) is not active.

With vsync and frame limit off, movement appears to track perfectly and accurately, but as soon as the frame rate is limited to below the level the GPU is capable of, movement appears to track less well.

This is a subtle effect. If the player always plays with vsync on, or vsync off or with the FPS limit set to the same value all the time, they will likely not notice anything amiss.