I run Drupal on some of my sites and vBulletin on other sites. I am planning a site which will use both Drupal and vBulletin.
I did some research and found vBDrupal. vBDrupal is a minor fork of Drupal which adds tight integration with vBulletin.
Yes, open source and commercial software sleeping with each other.
This would allow me to share user databases with both applications, and enable me to implement plugin's designed for either application.
Then I could implement the WordPress/vBulletin bridge and have a true best of breed solution!