Black borders on sides of 2nd external monitor with new Macbook Pro 15" Retina

Black borders on sides of 2nd external monitor with new Macbook Pro 15"
Retina

I just began using a new Macbook Pro 15" with Retina Display running
Mountain Lion. Since it has 2 thunderbolt ports, I decided to try hooking
up two external monitors. Both monitors are the same Dell model (24"
diagonal, 1920x1080 resolution).
I hooked up the first monitor with a Thunderbolt-to-DVI adapter and the
picture looks fine. I hooked up the second with a Thunderbolt-to-VGA
adapter and the picture on this monitor doesn't fill the screen. Instead,
there is a black vertical border about 3/4" wide on both the right and
left edges of the screen area. Also, I had to manually set the resolution
to 1920x1080, as my Mac defaulted to 800x600. This didn't happen with the
first monitor I hooked up. I checked the monitor for settings that would
allow me to adjust the picture size and perhaps expand it to fill the
screen area, but there don't seem to be any.
Any ideas on how I can get the second monitor to use the full screen area?