I can't really give you an apples/oranges comparison of Wordpress, VBulletin and Drupal because their intended purposes differ. You need to assess what you and your site need and base your conclusions on those requirements.
Wordpress is mainly used for blogging. Yes it can be scaled up, widgets added for varying purposes, etc. The fact still remains however, that Wordpress is geared towards blogging. Wordpress could be considered a 'Specialty CMS' as it does one thing particularly well.
Drupal on the other hand, could be referred to as a 'Full CMS'. A Full CMS effectively handles huge amounts of information and organizes it accordingly. Full CMS' allow you to not only blog but also implement forums, shopping carts, messaging, etc. In addition, Full CMS' have an administrative interface much more intuitive than that of Wordpress.
VBulletin is primarily forum software. If forums are the premise of your site, VBulletin may be a viable option for you. Since VBulletin is not free software, you may want to research other options if you do not have a large membership to your forum. (You can always port over to VBulletin if your forum becomes quite large.)
In short, Drupal will let you blog AND have a forum section to your site all in one tidy little tool. Wordpress is best for blogging but can be scaled up with widgets and the like. VBulletin is an excellent choice for forum software if that is what your site primarily consists of.