I have a laptop with Vista installed and no Vista disc. I've made a recovery disc using the recovery utility ripped from the beta version of Vista and the beta disc; I'd rather not have to test this.

I've installed Debian onto an external USB HDD which works fine.

Booting up to Windows or Debian isn't a problem, so long as the external drive is plugged in.
Booting up is impossible if it's removed. Grub loads and gives the "Resource not found" and gives the rescue prompt.

Nothing I've tried so far (which has more or less been shotgun debugging with the help of Google) has helped. I have Boot-Repair disc, but the best I managed was have it always boot to Windows (ignoring Debian); changing the boot order did nothing. The recovery disk finds no problems when repair mode is used.
I've tried purging grub, then installing to the internal drive as well as the external (after purging again).

This appears to be a fairly common problem, but I've not found a definitive answer from somebody that has successfully fixed it.
Current boot order:
Upgrade Bay
CD/DVD Drive
USB HDD
Internal HDD

In short:
I want to be able to boot into Windows when the external drive is not connected. I want to be able to boot into Debian when it is connected (being able to boot into Windows in this case would be nice, but not necessary).
Can this even be done? What do I need to do with Grub to achieve it?

That would be the behavior one would expect if when installing Debian on the external drive, you installed part of the Grub bootloader to the master boot record of the internal drive, the one with vista.

Install Debian Grub to the mbr of the external rather than the internal drive.

You will need to get windows bootloader on the master boot record of the internal drive. AFAIK, the files needed to do this are not available on a vista Recovery disk. You usually need a full installation CD/DVD but you could try the link below, review the instructions and click on the link below Option 2, the link for neosmart. You should be able to download an iso there to burn to disk to use. Instructions are on the site below:

http://www.bleepingcomputer.com/forums/topic347632.html