How to set the Display aspect ratio to 16:9 explicitly when using a VGA adaptor
Solution 1:
In the monitor/display settings just press the Option key when selecting the "scaled" option. Then additional resolution options will show up.
Solution 2:
Displays under System Preferences is it from the OS X built-in UI side.
There are third-party utilities like Display Menu that offers additional resolutions.
There is also cscreen if you want to do it in Terminal or incorporate it in a script.