Not really an Outlook question, is it? Have you asked in a Vista Windows
Mail group? As far as we can tell the Outlook developers know nothing and
care nothing about interfacing with other with the other email products that
Microsoft provides. They are on separate planets. While I seriously question
why you want to migrate from a robust email product to an anemic OS add-on,
it should still be your right to do so.
Assuming that transferring Contact data from Outlook to the operating
system's baseline messaging system is your wish, you would need to export
that data to a format the operating system can import. CSV seems to be the
lowest common denominator for that task. Expect and enjoy any data loss that
may occur in that process.