I have ubuntu 12.04.2 LTS AND i recently updated my system. There was a kernel update also. I tried several solutions on net. Like This , this and none of them worked for me.
I put that acpi_backlight= vendor and i was not able to change brightness at all. I removed that from grub file and i was able to change brightness from brightness and lock, function keys are still not working for brightness while volume function and other function keys are working properly.
uname -a
in a terminal.