I think you need to restore the Vista boot record on the hard drive (as if Ubuntu and the removable drive don't exist).

Maybe Grub can do this.

I understand that you can make the repair with a Vista installation DVD or a Windows Vista Recovery Disc

If you have done it right then Grub should have been in the MBR of the external hdd. Therefore removing it will force the Bios to boot whatever is in the internal hdd's MBR which should be the one left untouched by Vista. I believe you can still achieve it by

(1) While in Ubuntu check if the first boot disk is indeed the external HDD. You can compare the output of
Code:
Sudo f disk -l
And
Code:
Sudo grub
Geometry (hd0)
Quit
If sad is indeed the external disk (hd0) then you can duplicate the MBR from internal disk to external disk by commands
Code:
Sudo grub
Root (hd0, 0)
Setup (hd0)
Quit
Assuming you only have Ubuntu in (hd0) and the first partition has Grub inside which should be the normal case.

(2) You can restore the Vista's MBR by following the Task B7 of the just booting tips in my signature. Both the lazy way asking the Vista or Win7 installation DVD to do it automatically or manually using the command "boot /fix mbr” in Command Prompt should work for you. The command "boot /fix boot" should be unnecessary in your case.

Thereafter your wish will be granted.