As I remember it's not easy to change VPinball key assignments, since a lot of them seem to be defined in the scripts for each game. So you'll run across a table that has key assignments "hard-coded", including the "Enter" key for plunger.
Then again, what's the problem with having a button press "Enter" if you're also generating "Alt" "Ctrl" and "Shift" for MAME?

When I first built my cab, it was running fine with two U360's using the gamepad inputs for my buttons. I had no issues with MAME and other emulators, but I found to run VPinball I needed to buy an IPAC to generate the keyboard inputs needed for plunger ("Enter"), flippers (left and right "Shift" buttons), bump buttons (Z, /, and space, if I remember), and also coin and start (5 and 1). I could have used XPadder or joy2key, etc, but I couldn't get that set up reliably.