Can you please confirm if the Ernie Ball VP Junior 25K
is compatible with the Mastermind?

I have not personally tried it, but I don't see any reason that would keep it from working. You should make sure that the Mastermind is updated to firmware version 1.3 - you'll probably want to use the calibration feature to make sure that you get a full sweep with the pedal - lower impedance pedals need calibration more than the higher impedance ones.

I have a 25k Ernie Ball VP Jr. connected to my Mastermind and it works like a charm. Just remember to use a Y/Insert cable where the tip goes to the EB input and ring to EB output.