Please note that the Audio Unit plugin format does not support MIDI Learn on effect plug-ins so you will have to work with Smart Controls assignments:
1. Load your UVI plug-in as an AU effect in a Logic track (for e.g. Rotary)
2. Select Show Smart Controls from menu View
3. Click on the Information icon at upper-left corner
4. Click on the wished parameter in the Smart Controls window (for e.g. Rotary Speed Mode)
5. Go to External Assignment, click Learn then move any controller on your MIDI device
Your parameter is now assigned to your MIDI controller.