Since I wanted to be able to control both my Synth and VL2 at the same time I've been measuring the internal resistance between the pins of the MP-75 so I could create a second button. The outcomes where these:
MP-75 Wiring Diagram
(This links to an image on my facebook. Click & choose 'Open')
So basicly you shortcircuit between the 2 hot wires and the ground. If you would simply shortcircuit between 1 hot wire and the ground it wíll switch, but you'll also hear a loud pop. I guess this is because of the balanced signal. What the exact function of the 2 resistors is, I don't know. But I guess they're there for a good reason.
I've build a switch myself according to this diagram and it worked when attached to a cable between mic and flooreffect. But it won't work attached to a cable that's between 2 flooreffects. For example: I have this signalpath.
MIC --> VTSynth --> VL2
While modifying the cable between the MIC and VTSynth will work. Modifying the cable between the VTSynth and VL2 won't. The VL2 will just not register the switching in this setup.
So in the end I took apart the VL2 and directly solderd some wires on the Step&Shortcut buttons so I could connect 2 external switches to them.