Mini Thunderbolt to VGA not showing video on external monitor but Mac says it is
The problem that you are facing is simple to fix. Actually there is no inbuilt Thunderbolt to VGA drive inside a MacBook/Mac mini. So what you should do is install a Mini DisplayPort to VGA Firmware Update v1.0. After installation restart the system, then connect your thunderbolt to VGA connector. It will work perfectly fine.

Here is the link for the driver where you can download it:

https://support.apple.com/kb/DL799?locale=en_US