Ok you already have vista good. Now what you need is XP.
1. Fire up XP set up and install it into an unoccupied partition.
2. Once you finish XP instal you will not be able to get into Vista.
3. XP will write its own MBR (Master Boot Record) Which is still completely different from vistas so you don't have to worry about losing the Vista boot info.
4. No pop in you vista CD and go to repair mode.
5. Once there you will se the option to fix boot record for vista. Run it.
6. Restart you PC it will boot vista only (unless it somehow detects xp)
7. Once in Vista download a program Called VistaBootPRO.
8. Open VistaBootPRO, ignore the prompt to backup your BCD. Go to the System Bootloader tab, select "Windows Vista Bootloader" in the first section and "All Drives" in the second section and then click Install Bootloader . Next, go to the Diagnostics item on the menu bar and select "Run Diagnostics". VistaBootPRO will default back to the Manage OS Entries page and you will see that you now have entries there for "Earlier versions of Windows" as well as "Microsoft Windows Vista".
9. Restart and you are done.