If you don't get a j-pac you will have to hack a lot more than the vga cable, all the inputs for the controls will have to be cut from the jamma loom.
Using a j-pac it simply plugs into the keyboard&vga connectors on the computer and connects to the jamma fingerboard interfacing them all without the need to hack anything, thus leaving your jamma cab intact incase you want to use real pcb's.