Provided you have a free partition or extra disk to install Vista to then yes you can do this but not all XP drivers are compatible with Vista. Saying that RC1 does have much improved driver support than previous betas.
Also Vista has a new boot loader that does away with the ntldr and boot.ini method of booting and amending this has to be done with a horrible command prompt utility called bcdedit which is not for the faint hearted. But this can be overcome by using a neat little program called Vista BootPro which works well click here
Remember that when you delete Vista its boot sector will remain on the root of C: in a folder called Boot. You will have to delete this and then use the XP Recovery Console commands of fixboot and fixmbr to have XP boot automatically.
Hope this helps but feel free to ask more if unsure.