Im not sure about the import features with xenforo and what it exactly does. Does it download all data from the other database i specify or does it replace the current database i input on xenforo import features?
The import will copy an existing vB 3.7/8 database into your XenForo installation.
So essentially if you have a site with 500 members, 2000 threads and 10,000 posts, after importing to XenForo the content will be replicated and you can then continue using your existing site with the XenForo software.
Your existing vB database and installation will be unaffected, however, ensure you take a backup first.