I recently installed Mint 14 Nadia on my Sony Vaio SVT131A11L and have been experiencing similar problems not being able to adjust my brightness using either the menu or the Fn + F5/F6 keys. I tried GRUB_CMDLINE_LINUX="acpi_backlight=vendor" as per a suggestion on another forum as well as GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor acpi_osi=Linux" as per the suggestion in this forum. The first suggestion resulted in no changes, while the second suggestion resulted in the Fn + F5/F6 keys working, but only for one brightness level, and even then the brightness did not appear to visually adjust. I still cannot adjust using the menus either.
This is my first experience using any version of Linux so I appreciate any help I can get.