External display black on rMBP 11,1
You have a solid troubleshooting technique, so I'd agree the adapter, or the VGA cable are  likely culprits. Sometimes it's as easy as the cable not sitting well in the connector, and that could mean it connects the necessary pins for detection/sync but not for actual display. 

One other thing I could think of is if the monitor has a source selector, i e if it has multiple inputs for VGA/DVI etc, and the right one isn't chosen. That's probably not the case though as you say it responds to resolution change.