I have same problem on our older HP C700 Notebook using KDE (Gnome has a brightness applet)
Work around for us is to adjust brightness using the fn plus F8 keys on keyboard to increase brightness
Don't know if the same key combination is the same on the Sony vaio but there should be some kind of adjustment on your keyboard for the lcd backlight brightness
When this bug first appeared I did a google search for lcd backlight brightness at that time there was no info for our C700 try a google search for lcd backlight brightness sony
Desktop ASUS M2N-MX SE Plus, AMD 64 X2 Dual Core 5200, 4 Gb RAM, GeForce 9500 1GB
Notebook HP CQ61-311AX, AMD CPU M320, 4 Gb RAM, Radeon HD 4300 512 MB
Wireless AR9285, Audio ATI SBx00 (IDT 92hd75B2x5)
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