Oh, I see how namco did this. My vga monitor actually has a vga out. What you have is this wired for Jamma with the vga from the board to a finger board to jamma. From googling around, it seems your monitor may be switchable from stand res (15khz) to medium res (25khz). Should be a simple switch on your chasis board (high is medium res and low is stand res). I would recommend staying in 15khz res as this is the res for 90% of the games out there.
so.........I would suggest these two options
#1 You get a J-Pac and ArcadeVGA card. This is the easiest option. All the buttons are already wired in the jamma and if you wanted to add extra buttons, you simply wire and screw the extra ones to the j-pac.
#2 You find the wires from the monitor (red, green, blue, sync, ground) and connect them to a vga breakout cable from ultimarc. You then plug the vga cable into your video card on your computer. You run soft15khz software to switch refresh rate to 15khz. You need to make sure that your video card will work with soft15khz though. I can tell you that my ati x1300 card works perfectly with soft15khz. You still need a J-pac.