"How can I automate this button...."
Three ways. 1. In Basic View, right-click on the button itselt. It'll blink. Move any connected controller, and it will automatically be assigned to that CC
2. In Advanced View, select the "Ctrl Map" tab, then select "VelXF On/OFF" and under source, click the drop-down menu and select a CC.
3. Much like 1, in Advanced View, select the "Ctrl Map" tab, then select "VelXF On/OFF," click "Learn," and move the controller you want to assign.
If you select a continuous controller, any value above 64 turns the button on, and under 64 turns it off.
The button acts as a kind of clutch to disengage Velocity Crossfade, as musically needed.
Yes, there is an investment of time with VSL products. I've been using them for fifteen years or so, and I still need to review the manual now and then. A paid tutorial site called Groove3 has two multi-episode tutorials.