Setting resolution to 1920x1080 for external monitor

I just purchased my first Mac. It's a MacBook Pro 13 with Retina display. I have a new LG monitor and am using a VGA adaptor with Thunderbolt. When I am hooked up to the LG monitor, I cannot get the display to show 1920x1080 .The monitor recommends this as the best option, but cannot find where in my MacBook settings to change this. The only two resolution options available to me in the System Preferences -> Display settings are 1280x1024 and 1024x768. Is there a way to manually add a third (or more) resolution options? The screen on the macbook itself is fine, just not displaying properly on the external monitor.
Thanks!