You're having the same problem I'm having. Most folks have been able to remedy it with an uninstall/reinstall of Mappoint. I unfortunately have not been able to get Nav to work on Vista no matter what I try. I can't get my system to recognize the SD Mappoint plugin under Vista.
Try the following:
1. Make sure UAC is disabled.
2. Enable the Administrator account of your pc.
3. Login as Administrator
4. Open Mappoint
5. Go to Tools - Addins
6. Make sure the checkbox for Streetdeck Mappoint is checked.
7. Hit Ok, and if you don't get an error message, you should be good to go.
Log out and log back in as your other account and you should be set.
Unfortunately this did not work for me. But it worked for others.