It's already passed upstream if you take a look at it.
Are you sure it's: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" acpi_backlight=vendor and not: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor" ??
For me the latter one works.
It's already passed upstream if you take a look at it.
Are you sure it's: GRUB_CMDLINE_ LINUX_DEFAULT= "quiet splash" acpi_backlight= vendor LINUX_DEFAULT= "quiet splash acpi_backlight= vendor" ??
and not: GRUB_CMDLINE_
For me the latter one works.