Are you still having the same problem? Is the adapter an Apple product or third party? I am considering an AOC 24-inch display.
Analog VGA is not a good choice of protocols for a display made in the last two years.
You should use, in order of preference:
DisplayPort or Mini DisplayPort
*If your display is wider than 1920 wide (1080p is 1920 by 1080) you will need more expensive Dual-Link DVI or HDMI to support doubled data conductors in the cable.
Haven't had this issue in a long time. I have a feeling they fixed it in an update. But I do still have the rare instance were my secondary display will randomly change aspect ratios. I will be working and look up all the sudden my screen looks like total crap and its zoomed it. I just have to unplug and plug back from the thunderbolt adapter. It's rare though.