The instructions in
/etc/default/grub say to run
# The resolution used on graphical terminal # note that you can use only modes which your graphic card supports via VBE # you can see them in real GRUB with the command `vbeinfo' #GRUB_GFXMODE=640x480
but that command isn't found in GRUB,
grub> vbeinfo error: can't find command `vbeinfo'.
or in the terminal:
$ vbeinfo vbeinfo: command not found
Where is it?