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.
Just to confirm, you tested both acpi_backlight=vendor and acpi_osi=linux (together and independently) and ran sudo update-grub after each attempt? Also, you might want to mention the kernel version (post the output of uname -a) as well as hardware information.
Try this app .. https://apps.ubuntu.com/cat/applications/brightness-controller/
I was also searching stuff related to driver , but at last I found this app . I am having ubuntu - 12.04 on Sony - vaio, working fine for me.