XenForo now has an official vB4 importer included with the software (as of version 1.2b5). You should use the official importer from now on. But this unofficial importer is still useful for the extras that it includes (social groups, user albums, blogs, CMS articles). These things are not included in the official vB4 importer. The official importer ignores data for these features which XenForo does not have, while this unofficial importer converts that data into threads. This unofficial importer also has a "post thanks" module to convert "thanks" from that popular vB4 addon into XenForo likes.
Adam posted a copy of the previous vB4 importer that he used to maintain so I have uploaded it as the current version. This version is more up-to-date and fully featured than the old one I posted before. It imports everything including social groups, user albums, blogs, and CMS articles. Since XF doesn't have those features it imports them as threads / attachments.
Note that you only need to import the XML file if you are using the "post thanks" import module. If you won't be using that module then the XML is not needed... just upload the files and run the import.
Old Version (available in version history):
This is a repost of the importer from this thread:
It contains files that need to be uploaded to the /library directory. The importer will show up in the Admin CP after you upload the files:
Admin CP -> Tools -> Import External Data
As you will see from that thread, marioman posted the original importer and then svetoslav80 posted some updates to include social groups, blogs, albums, and CMS articles. svetoslav80's copy of the importer is included in this package. If you need svetoslav80's version then you need to rename these files (remove the .extras):
Otherwise marioman's version works great for the forum stuff. I have used marioman's importer many times so I know it works. I have not tested svetoslav80's version myself, but he's got a few testimonials:
Also note that this importer includes an addon file (addon-vb4ToXf11.xml) which must be installed if you want to use the "post thanks" import module (it's a vB addon that some people use). The XML file can be imported in your:
Admin CP -> Home -> Install Add-on
After the import you can use these redirect scripts (yes, they apply to vB4 as well):