So in MSFS, Control Options, select the Honeycomb Alpha.
Search by assigned and press the button you want to use, if it’s assigned (it is by default) un assign it.
Then, open the vPilot Client, go into Settings, Push To Talk and assign it that button.
Basically you want to un map it in MSFS so it’s blank, and then map it in vPilot for VATSIM.