Brightness settings gone, how do I bring it back?
Solution 1:
Navigate to Start -> Control Panel -> Hardware and Sound -> Device Manager.
In the Device Manager window, look for “Monitors”, then right-click your monitor and select “Uninstall”. Be sure to also check the “Remove driver” option. Once removed, click the “Scan for hardware changes” icon. Doing so should bring the brightness settings back.
For both NVIDIA and ATI drivers for Sony Vaio, after performing this solution, you can restore the brightness via the Mobility Center (Windows+X).