Portrait display resolution issue since upgrading to Mountain Lion

Hello. I'm running a Mac Mini 2011 with the 2.5 GHz i5 and Radeon HD 6630m graphics. I run two monitors, one via HDMI and via the Mini DisplayPort to VGA adaptor. The HDMI connection runs landscape, the VGA portrait. Since updating from Lion to Mountain Lion, the resolution options in portrait disappear. I'm forced to use 800x1200, which stretches and distorts horribly, as opposed to the correct 1366x768 available to me in landscape.

Prior to updating from Lion to Mountain lion I didn't have any issues, so it's pretty clear this is Mountain Lion related. I'm hoping someone can help me resolve this issue.