If you look at the screenshot, it’s saying Button 4 on TCA Q-Eng 12 - That’s the throttle quadrant, not the joystick. Something is probably continuously triggering that on the quadrant, and that’s why it was assigned on vPilot after pressing “Set a PTT button”, and why you have a hot mic.
This is what mine looks like, using the trigger on the Joystick
If you still can’t set it, try disconnecting the throttle, set the PTT on vPilot, and reconnect it