Hmm, the licensing clearly states that you can only install one of the two OS's, hence UPGRADE. I don't know the exact wording, but I do know because I went and found out when I wanted to know. It's in the EULA anyway.
Outwith the licensing aggreement though, theres nothing stopping you installing in on the same machine on two seperate partitions. Just as long as you know what your doing and that you are confortable with it.
And just one point, why would you want too? 7 is designed to be completely backwards compatitable with Vista, hence the reason why the kernel is even labeled as version 6.x.xxxx, like vista, yet it's hugely different. This is only so that programs checking the version of windows thinks it's the same family as vista.
I see the point in keeping a XP installation somewhere, just incase you've an old App that don't work or a 16bit oldie program you need to use. But not Vista.
Thats my 2P. Best of luck, lol.