If I remember correctly, you actually export the XP address book to a file
and then you import it into Vista by way of that file. (If you've ever
exported/imported browser bookmarks, it's a similar process.) It's not like
you can point Vista to the XP drive and just yank the addresses out of XP.
At least I don't think so.
Anyway, I just jumped in to give you an answer until someone else responds.
I KNOW there are more knowledgeable people who will give you better
instructions than this.