Unfortunately, we do not currently have a vBulletin import script for Gossamer Forum. It was not a high priority for us when we were writing the initial import scripts because our thought was that most existing vBulletin users enjoy the mass multitude of options with that forum, which is not the focus of Gossamer Forum (at least not in the version 1 product cycle).
A few users have managed to transfer the post data by first importing into another supported forum program (such as ikonboard or WWWThreads), and then from that program into Gossamer Forum.
Jason Rhinelander
Gossamer Threads jason@gossamer-threads.com