I would agree with past posts about not using VGA, even though there's a 15 pin VGA port on the plasma panel. The primary reason is that you won't be able to access all the display modes that the iMac offers and likely won't be too happy with the aspect ratio of the screen. With the displayport to HDMI or DVI (if your panel has a DVI port), then you'll be able to get the correct aspect ratios.
I struggled with my Mac Mini and a Panasonic 55" plasma and even SwitchRes X couldn't make VGA output work. Going to HDMI turned that panel into a great monitor.