Again, not quite. Please read and try to understand this....
http://support.microsoft.com/kb/919529
The only sensible way to dual boot XP and Vista is to install XP first, then add Vista. Vista uses a new boot process not compatible with the XP boot process.
If you remove all drives and first install XP on your new large drive, you could then add whatever old drives you want, reformat them, and then install Vista on them, telling it you want it on C: or another drive D:, according to preference. What will
not work, is installing XP on a new drive, then adding the old drives with Vista already installed, and expecting Vista to appear as dual boot option - it
wont happen.
However, the link given seems to imply that if you have a working Vista, but then install XP (destroying the different Vista boot), it is possible to recover the Vista boot option by booting into command-only mode, and running a program found on the Vista install CD. What you can do depends upon what install media you have.