I would highly suggest going with Xenforo over Wordpress... Wordpress is not a forum, it's a CMS. This site is Xenforo, and is a prime example of how extensible and powerful it is. It also has a built in importer, which is compatible with your version of vBulletin. Wordpress may be free, but the nightmare you would face trying to make it a forum, is in no way worth it.
Converting vbulletin into xenforo is fairly straight forward. Setup xenforo, note the database info for the vbulletin forum, click on import and enter in your vb info.. Click continue import about 12 times. Rebuild your search index and wala!