1) The unofficial vB4 importer does user albums as threads:
http://xenforo.com/community/resources/vbulletin-4-to-xenforo-importer.256/
The official vB3 importer (included with the software) doesn't do albums at all.
2) Yes and yes.
3) The minor version doesn't matter, only the major version (3.8.x).
4) The importer is very thorough. But one mistake I often see is people who do a test import, develop their forum using the test install, and then expect to be able to run the import again and take the test forum live. The problem with this approach is there is no easy way to deal with duplicate content when you run the same import twice on the same XF install. You should think of the test install as temporary and use it only as a point of reference in doing your final import on a new database.
5) Most tablets do not require a special style. But here is a mobile style that is optimized for mobile browsing:
http://xenforo.com/community/resources/xfs-mobile-style.290/
6) Paid subscriptions are not imported from vBulletin. They will need to be recreated. There is an unofficial importer but I have discovered a few problems with it:
http://xenforo.com/community/threads/vbulletin-4-subscription-importer.19169/
