Somebody has written the attached hack for vBulletin. (Thread in Vbulletin Forums).
The main script (new_news.pl - yes, in Perl ) that synchronises usenet with Vbulletin contains nearly all of the functions that you'll need to integrate Usenet with Gforum. As such, the vast majority of work is already done for you. The main tasks would be rewriting the database inserts to work with Gforum's table system, and writing the front end for Gforum. If you can translate the php into perl it shouldn't be too hard.
It would be great if you could release this plugin if you finish it.