Many video cards do NOT support 15KHz, or if they do, they have to be controlled at DOS level instead of via the windows driver. (Thats why the special 15KHz cards are popular ad ons)
Be aware, simply connecting the VGA signal to the monitor will result in a very dark picture. Arcade monitors require a 1-5 volt signal range. VGA monitors/cards use a 0 to .7 volt signal range.
Therefore you have to crank up the brightness/screen controls (causing other monitor problems) in order to see anything. You will need to put in some amplifiers on each of the 3 RGB lines to give the arcade monitor a usable signal.