If your VGA display exhibits reduced brightness with your Mac mini, here's how to troubleshoot the issue. If your Mac mini won't display video at all, see the "Troubleshooting: Mac mini displays no video" article.
If your VGA display exhibits reduced brightness when connected to your Mac mini, try these steps to resolve the issue:
Make sure that the display is turned on and has power. Make sure that the display's power cord is plugged into a working wall outlet, and that it's properly connected to the power adapter. (To check if the wall socket is working, plug in a lamp or other electrical device.)
Make sure the brightness and contrast is set appropriately for your display. Some VGA displays offer additional controls. Make sure that these are set appropriately. (See the manual that came with your display or contact the manufacturer to find out how to do this.)
Check that the video cable connections to the computer video port and to the display are properly connected. Remove any extensions or switches for testing. Reseat the cables firmly and make sure the connectors are secure.
Examine the pins on the video cable connector to make sure they are not damaged (either bent or recessed lower than the level of the other pins). A small bend can make a big difference.
Calibrate your display using the Display Calibrator Assistant. Follow the instructions in the assistant to create the best profile for your display. Make sure that the Expert Mode box is checked.
If you tried all of the steps above and the computer still won't display video as expected, contact Apple technical support (1-800-APL-CARE in the U.S.) or take your computer to your local retail Apple Store or Apple Authorized Service Provider (AASP) for diagnosis.