Simple question I think.. Can I just add a second disk (my old XP system disk) to my vista machine and use Vista Boot Pro to select which OS I want to boot from?
Re-installing the either OS is not an option at this point.
I have been trying for quite some time with little success, and many of the suggestions on this thread point to reinstall the os.

This guide should help you
http://www.pronetworks.org/index.php/guides/post/unplugged_xp_drive_for_vista_setup_cant_dual_boot_to_xp/

The first thing I am going to ask is the XP image from the computer you want to install it on?

I assumed they were from the same computer, thanks for pointing that out Jeff

No. My XP machine was a few years old, but i was running a licensed version of ProTools (can only be installed 3 times which i exhausted reconfiguring the old machine). Last fall, i purchased a brand new quad core machine running Vista.
I wanted to take the HDD out of the old XP machine and install it in my New Vista machine as a secondary drive. then use Dual boot to be able to continue to use my Protools, but on the newer more robust machine.

Wow that is some expensive software, I can see why you don't want to pay for that again. Have you tried contacting Digidesign and see if they will help you out?

No I have not. I am pretty sure they would work with me, but there are other issues. The interface hardware and the version of protools i am using are incompatible with vista, and i was reaaaallly trying to avoid the (install xp again, download all the serv packs and security patches, install antivirus software, and all it's patches, all of the misc software, install ProTools, download and install all the plugins) hours/days of work involved.
I've been a pc/lan support tech for 10+ years and have used dual boot for many of them... I just can't believe Microsoft has made it such a problem with Vista, which is why I turned to you. Microsoft gives so little info on BCDEdit and troubleshooting... argh!

I have extra disk drives.... do you think this would work?
new machine: install the xp os disk as the primary drive make sure it boots up fine, and install a blank hdd.
install Vista on the new blank hdd.
copy the boot files from the new "Vista" os disk over to my previous "Vista" image.
Swap out the new and old vista imaged disks.

Then use Vista boot pro to manage booting...

I am probably reaching, but am fairly motivated at this point to make this thing work.... I just hate getting beat by a computer.

It is not that Vista is a problem with dual boot, in your situation exactly. The problem is with the XP drive being from a different computer and thus having the drivers and what not set up for the other MB and hardware.
If you would have had the opportunity to setup XP first on the computer that now has Vista on it, and then installed Vista, it would most likely have recognized that XP was there and included it in its bootmgr, and you would be dual booting as we speak, actually we probably would not have met (-:

To get back to what you are proposing; I believe you could skip the step where you install Vista on a blank or empty drive etc. and follow the guide that I first posted, that is provided you remove the Vista drive during the install of XP on a new drive.

It's just pot luck in your case, really -- but be aware that the odds are heavily stacked against you.

The most likely outcome is that the installation process, porting the HDD to a modern computer, would render your previous XP installation completely unuseable.