[Lynne]

There is no vbulletin function to do this. You could try deleting them all via vbulletin (users and posts) and then resetting the auto_increment property for the tables in the database. But, if you don't delete everything, and don't reset them all, then you could cause problems. I'd suggest if you are going to do this, you do so on a test site and take notes on what you did so you can do it right on the live site.

Templates/Styles/Languages can all just be exported and then imported to the new site. Forums cannot, unfortunately.