If you unplug your Vista HDD while installing XP you wont have this issue. Thing is you have to make sure that that HDD is set to master. Then when you plug in the Vista one again that has to be set to Master and the XP drive to slave. If you forget the system will try to boot both OS's and give you a error.
You could use the Cable Select option as well. But that will give you Vista and not XP. At that point you will have to unplug the Vista drive. Plug in the XP drive. Boot it. Go to Start>Run>msconfig>Boot.ini tab and write down you boot info.
From there plug in the Vista drive again and boot back into Vista. Easiest way to edit the BCD is with EasyBCD. So use that and edit your BCD to include the boot for XP. Then you will have a dual boot system.
Only problem with that is XP will think it is hte first partition on the primary HDD. Same as Vista. So you could run into issues.
Easiest thing to do. Put in the new HDD. Format it. Insert the XP CD and isntall XP on that drive. From there do above. Go into the msconfig and write down your boot info. Use the VIsta CD to repair your boot config. Go back into Vista and use EasyBCD. Add hte option to boot to XP to the Vsita partition. Saves all the time opening and closing your case to dual boot.