As one of the contributors on the vbulletin importer, the author of the VB 5 importer and someone who did over 50 vB to Discourse conversions, I was just sharing the information that the importer is mainly tested on vB 4, not on 3.8 which is EOL for over 7.5 years now. I was not trying to “impress” you…
Anyway. I wonder if this is a bug in the importer, a vB 3.x related issue, or a broken database. I suspect the latter options are more likely. You could modify the code to select a default category in case the detect function does not return a result, but (at least) on vB 4 this would indicate a dangling category without a proper parent, i.e. a broken db.