IF it's JUST the forum, no blog, no cms, no user groups, no gallery, etc, it can be done, I did it just the other night. And I used the built in importer that is for the 3.7/3.8. You can see mine here: Secret City Designs Forum
Be sure and clean your vb forum well and that your user name in the vb site matches the XenForo site as closely as possible prior to import. And move all your members to one user group except for your moderators. Example, if you have more than one user group for regular members, move them to one group called Registered or Members. Get rid of all banned members.
But I did it and did it without the 4.0 add on script.
No, it isn't but I didn't want to carry mine over. However, most of mine aren't attachments. Mine were typically attached to the server anyway, Moving them to another area on the server, didn't affect mine. And my users typically used photobucket.
If someone is running a vb 4 forum or even a suite that is very basic, then the built in importer will work following what Rob and I have done. But we are the "zebra's". We aren't the run of the mill vb 4 site out there. Most vb 4 site's need your importer, just not 100%, more like 97%. So you still are there for most people. What Rob and I are saying is that it is possible.