Are the individual columns utf8 as well? (Normally they would be, but they can be different.)
Forcing the character set when starting the import to "utf8" should be sufficient then.
If that still fails, I can try to debug it if you submit a ticket with FTP and admin CP details. I'd need you to start the import (insofar as entering the source details and getting to the step list). To debug quickly, if you can provide the ID of a thread whose title ends up blank (the ID of it in vBulletin), that'd help. I can then test directly against that.