A bit late on the response , I'm afraid , but here's my take on it:
It looks like grub tries to look for the Vector-kernel at the wrong place.
If you have any means of booting up your other distro (debian , right?) do so and edit the Vector-entry for Grub , so it points to the full path
ie.- If Vector is installed on hda5 , the entry should read something like:
image = /boot/vmlinuz
root = /dev/hda5
label = Vector4.0
This is taken from my Lilo.conf , might look different in Grub.