I'm sure in the nearest future we will see a lot of modifications ported from vBulletin to XenForo. Users will reasonably wish to save their content, so they need an importer to do that. The simplest and the most logical way seems to be an extending standard vBulletin importer; so users may import modifications' data together with their members, threads and posts. However I can't see an easy and extensible way to do that now. We can hook into XenForo_Model_Importer and make it load our class that extends XenForo_Importer_vBulletin instead of loading XenForo_Importer_vBulletin itself, but when it comes to second custom data importer from another developer it's not going to work anymore. So I think it would be really useful to add dynamic class loading to importers and add load_class_importer code event.
It's great You should also consider allowing to import additional fields for standard content types (like some field in vb's user table). Currently fields which will be imported are defined (e.g. for importing user) in _importUser method which gets an array with user fields from vb's db, creates new array based on that first and passes second array to DataWriter without leaving a possibility to modify its content.