Thanks for your reply.
So I added a # on the line blacklist vesafb , but for what I have to do next : sorry I have no idea ! ( linux noob :-| )
So I found elsewhere what it seemed to be done :
echo vesafb >> /etc/initramfs-tools/modules
update-initramfs -k all -u
defoptions=quiet splash vga=871
in grub/menu.lst .
But at reboot, same problem... :-(
So maybe I missed something ?