Having worked with co-existing XP and Linux partitions, which is in principle the same issue, XP and probably Vista must have their way. You generally have to follow their trail of breadcrumbs to set up the boot (which is also the OS) partition.

You could do multi-OS boot with Mac OS X (for x86 I'm guessing) boot camp or a linux bootloader like Grub. Or if you just want to be done with the Vista test, do what they say to do with the XP disk and abort, and report here, if something seems to not be proceeding in a correct manner during the process.

Apparently it doesn't give you the prompt to load Vista or XP at startup?

A novel idea would be to load a trivial version of linux on it's own tiny partition, like Damn Small Linux, use it to install grub, and then boot whatever the hell you want to at will. I could point you to the relevant documentation and help you through it, but as no one has ever taken me up on these offers of assistance, I won't hold my breath.

Good luck.