The first thing right off is the Vista boot loader is totally different from XP. When installing Vista following XP the new "boot" folder is created on the XP partition or drive. A tool like VistabootPRO can then easily rename "previous operating systems" on the boot menu to "XP HOME" or "XP PRO" if you want to see it listed that way. You can also set XP as the default OS to load with the use of the BCDEditing tool there.
http://www.vistabootpro.org/intro.php
Presently I have been trying to find a way to restore the XP Home entry plus see XP Pro added in and multibooting with Vista. The entries were made but still nothing once XP Home was first removed from the Vista boot manager. Several articles on dual booting XP with Vista can be found with a quick search. Here's a few to look over.
http://www.windowstalk.org/dual_boot_vista.htm
http://apcmag.com/4054/how_to_dual_boot_xp_and_vista
http://www.pro-networks.org/forum/viewtopic.php?t=88193