Assuming you can boot Linux by changing the boot priority in the BIOS, you should be able to load Linux with GRUB installed on the external hard drive.
I assume you will also want to load Vista (although I have no idea why). To do this, keep the GRUB drive as the boot drive in the BIOS and add something like this to grub.conf:
map (hd0) (hd1)
map (hd1) (hd0)
Last edited by greeniguana00; 04-08-2007 at 05:43 PM.