It's possible but but but... you're likely to encounter issues because of how Vista's new bootloader works - it's very different than previous versions.
I doubt it'll be a problem but but but... there's always a chance the XP installation might not be bootable after it's done, and that's another story all together.
The preferred install method is:
- older version of Windows first, and preferably in order of chronogical age progression
- Vista after all the others are done and working properly
If you search **gasp** you'll find several threads asking this exact question, all made within the last few days... but....
Anyhow, there has been a method posted outlining ways to do this, but the results have been mixed, having worked in a few cases, but failing in most.
In addition, there is a program called VistaBootPro, and I believe if you visit their forums there is yet another method for installing a legacy os after Vista that has worked as well.