Apple - How to set the Display aspect ratio to 16:9 explicitly when using a VGA adaptor

In the monitor/display settings just press the Option key when selecting the "scaled" option. Then additional resolution options will show up.

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.

Display menu doesnt do 16:9 on a Retina MBP...only 16:10 and 4:3