You can't control XP with Vista's bootloader, unless you own a copy of Vista and you copy all the Vista files across.
You don't need the BCD however, NTLDR will multi-boot as many XPs as you want by itself.
If you installed XPH to the same HDD as XPP, it should have detected the existing system and created the dual-boot automatically.
If you installed XPH on a new HDD and it wasn't aware of XPP, you'll need to add a line to the boot.ini on whichever system is 1st in your BIOS boot sequence, to point to the other system.
It's described in the wiki, but if you need help, post a screenshot of your disk management display and paste the contents of the boot.ini on the XP you wish to be in control, and we'll advise.