Vbulletin DataBase Convert
Thank you for your interest in wpForo.
I'm sorry but there is no migration tool for vBulletin 3 to wpForo. At the moment I think the best way is asking to ProfProjects GP to do this job: http://profprojects.com/migrate-vbulletin-forum-to-wpforo/
Also if you can find vB3 to bbPress migration tool, then you can use bbPress to wpForo migration plugin:
Another good option is gConverter:
I'm sorry, but there is not a free solution yet. Both gConverter and ProfProjects provide paid services.