It depends on your motherboard. Some motherboards will disable the on-board graphics when a discrete graphics card is discovered. If this is the case, there may be an option in BIOS that will allow you to re-enable to on-board graphics.
If that is not an option, your current graphics card may be able to handle this. Just check to see if you have an available DVI-I output from the graphics card. If so, that output carries the analog signal that a VGA input requires and all you would need is the DVI-I to VGA adapter that comes with most graphic cards.
-Wolf sends