vBulletin users to WordPress site

We moved our clients existing vBulletin files and database from their old site to the new WP site we are developing. We simply embedded it into a WordPress page! It looks great, and all of the posts, images and user info is there. Now I need to figure out how these old users can log into the WordPress site and have vBulletin log them in as well- basically making it a seamless experience for them. Also want all new WP users to be automatically set up for vBulletin posting when they register. I have read about plugins, but hoped someone would have the most effective solution for this. Thanks in advance!