I just did the VSHG mod with the Cthulu. I sucked at just about everything so it took me a bit longer than most people and I never soldered in my life.
Looking back, The only thing you need to do with Daisy chain the 6 buttons and the Start Buttons together and send the wire to one of the ground of the Cthulu,
Pull the wires going to the VSHG main PCB from the the buttons and slide them into their corresponding hole on the Chtulu.
Solder 5 wires on the bottom of the Main PCB where the 5 wires coming from the secondary (R2, L2, select, home) PCB. Send these wires to their corresponding input on the Chtulu, the fifth (black one) going to the ground.
Find a place to Mount the Chtulu (i placed it at the corner on the left had side, where the joystick is.
If you don’t like the soldering option, I imagine you can either A. cut the wires coming from the second PCB and find a way to attach wires to it.
Or B. you cut 4 holes into the plastic and install your own quick disconnect buttons, daisy chain one side to the ground, and wire the other to their corresponding input on the cthulu.
I didn’t go the route of the MC cthulu so I have no idea. I modded mine so that I can once and for all eliminated the 3 button problem. Even though I had the latest release of the VSHG, it still occured in games like SFIV.
Hope that helped.