I fixed it! I used EasyBCD to add the menu entry for Win XP to the Vista bootloader, but there is an extra step:

You have to copy the ntlr, ntdetect.com and boot.ini files from the root of the XP drive to the root of the Vista drive, and then edit the boot.ini (through EasyBCD) to set the correct entry for the drive.

I had to change the following entry:
multi(0)disk(0)rdisk(0)partition(1)
to
multi(0)disk(0)rdisk(1)partition(1)

Hope it helps others with the same setup!