It's because Vista boots based on the disks it was installed with...like disk0 disk1 disk2 etc. If you connect more drives it moves around those physical locations. You're original disk1 could become disk2 or disk3 so when the vista boot loader tries to start it is now looking at the wrong drive....what you need to do it manually edit the boot location once the other drives are present....the only problem with that is that I haven't done it yet and don't really know much about it....bcdedit is what you are supposed to use to edit the boot loader but it doesn't really give you options as far as disks, only drive partition letters.
Best bet is to go into bios before pluggind in the other drives and checking the drive location....then once the drives are installed check where they are after and basically switch them around...if you have a decent mobo you can manually change their locations with SATA drives