I suggest removing the XP hard drive while installing.
Basically, Vista has a new bootloader (BOOTMGR) which has its own boot sector and MBR code, and it's incompatible with XP's loader (NTLDR).
When Vista installs, it looks for the first FAT/NTFS active/system partition and places its BOOTMGR and BCD files there. Meaning, unless you used diskpart to set the XP partition as inactive, it'll use that partition for the boot files. Meaning, that your XP hard drive will lose its original MBR/boot sector and it'll cause all sorts of problems.
On the other hand, removing the XP hard drive and having only one, the Vista HDD, set up to boot first (through BIOS), will make sure that everything is written to the Vista hdd, meaning that if the Vista HDD fails, you can boot directly from the XP hard drive and carry on.
Just pop open the case and unplug the data and power cables from the XP hard drive, then plug them in AFTER you've booted & set up Vista.
If you can't do that for any reason, install Vista as normal. When it asks for your product key, hit Shift+F10 to pull up a command window. From here, you type in (<ENTER> indicates pressing the enter key):
diskpart <PRESS ENTER KEY>
select volume C: <PRESS ENTER KEY>
inactive <PRESS ENTER KEY>
remove letter=C: <PRESS ENTER KEY>
exit <PRESS ENTER KEY>
Then close the command window, type in your product key and whatever else it prompts you for. When it gets to the partitioning/disk management screen, select the empty space, create a partitition (on the Vista drive), format it, and do whatever else, and install to that partition. Before hitting the "next" button, verify it's done correctly, using Shift+F10, then diskpart. Just type in "help" in diskpart for the commands.
This *should* preserve your XP disk, and still add the XP installation to Vista's boot loader. On the other hand, you can unplug the drive, install, and use VistaBootPro or bcdedit to add the XP boot entry later on (far easier).