vB User Copy plugin by Arimil allows you to copy users from vBulletin 3.x or 4.x. Before this plugin will work you must set the table prefix for your vB tables. This currently only has support for importing users from within the same database.
Users will be imported into WordPress with a random password. Upon their first login the plugin will check their password with the vBulletin user table. If the password matches it will rehash their password into a new WordPress phpass hash.
This does not integrate the two systems in any way, it simply imports the users from vB into WordPress.
Download and info: http://wordpress.org/extend/plugins/vb-user-copy/