If the (Vibrato) Sync switch is turned off:
Adjust the Via Amount slider to the right of the waveform graphic to set the maximum modulation intensity (the highest vibrato speed).
Move your MIDI keyboard modulation wheel to change the vibrato speed. If you don’t want to use your keyboard modulation wheel, click the Via pop-up menu to assign a different MIDI controller.
Download the guides:
Logic Pro User Guide: PDF