Odd that this happens after changing the menu key. Can’t really imagine how that might be possible, but I’ll take a look at it and see whether I can replicate it here. You will have to restart MPC-HC for the change to take to effect, just in case you didn’t do that.
If you happen to have a gamepad connected to your PC, try that instead to use the vorpX menu. Tapping the left thumbstick button will open it and then you can navigate it pretty much like most game menus are navigated via a gamepad.