In Windows 8 there is a featured called “adaptive brightness” which is often blamed for screen brightness settings going astray.
However, with Sony Vaio laptops the disaster that causes the automated screen dimming is a setting withing the Vaio control panel.
Put your finger to the right side of the laptop’s display & swipe it across the screen to the left slightly. Click on the “search” option & put “Vaio control center” in there. Then from that result list click on the Vaio Control Center listing.
Once it opens, you should see “image quality” in the left menu. Click on that.
There is an on – off slider for Display Brightness that controls this evil feature. It says “Set whether to sense the brightness and automatically adjust the display brightness accordingly.” … set that feature to off & your monitor darkness problems should be solved.
Once that is off, if the screen is still dark, you can hold down the function key & hit F7 a half dozen times or so to brighten it up.
At that point you should be all set with that brightening your screen. If it for some reason dims again, then you might have to go into your “power options” (by clicking on the battery icon in the lower right) to adjust the brightness of your screen under it’s current settings. There are multiple plan options for “balanced” vs “high performance” vs “power saver” … set your plan to high performance & verify it’s screen sliders are set to full bright when plugged in.
The unusably dark screen problem should be conquered at this point.