Hello John W Smith
Thank you for your interest in vBulletin.
The permission system in vBulletin is not compatible with that of UBB threads. For this reason you will always need to set the correct permissions after an import. In most cases this is a quick and easy job.
I am not familiar with the UBB Threads importer, but you can review the documention here:
ImpEx systems and more general:
Appendix 5: ImpEx Import System
A lot of information can always be found here:
vBulletin 3.5 Import System
If you have any questions regarding teh Import, then i suggest you ask at the
vBulletin Pre-sales Questions forum, to get an official answer.