If you are like me: “not able to select the right display resolution for your display”: go ahead an press the “OPTION KEY” while selecting “Scaled”.
This enables all (un)supported resolutions. The problem occurs for instance because of a bad EDID handshake of your graphics adapter and your display.
For me the problem arises using a “LogiLink Mini DisplayPort Female to DisplayPort Male Converter” which connects via an original APPLE VGA adapter to a VGA display. (Other than that, this is the only solution to be able to use a VGA display with the latest OS X versions BUY – http://www.amazon.de/LogiLink-Mini-DisplayPort-Female-Converter/dp/B002UTSNGC)