Hello. I also have a Sony Vaio, although it's an S series, and I had the same problem. Turns out that Sony Vaios have a feature to sense the brightness and automatically adjust display brightness. No matter what changes you make on the Windows 8 Power Plan, this feature will change them seconds later.
You can disable this accessing the Vaio Control Center, then go to Image Quality, and then set to OFF where it says "Display Brightness - Set whether to sense the brightnes and automatically adjust the display brightness accordingly". I got this from Vaio Support and worked nicely. Hope it helps.