Here is one way to map the ModWheel to the slow/fast switch
After adding MVintageRotary to the VB3 instrument or audio track
Select "Enable MIDI Input" in the VST2/3 drop down above the MVintageRotary UI
Click the MIDI button on the MVintageRotary UI
Enable the Controllers Tab
Click the Parameter drop down
Select the MIDI slow/fast mapping
Click Ok to close the dialogs
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Add a MIDI track, input set to your controller, output to MVintageRotary, input echo on
VB3 has a leslie with the slow/fast already mapped to the ModWheel. Either turn the leslie off and/or add the MIDI Event filter to the MIDI FX rack of the VB3 and filter out the ModWheel