vBulletin integration has been discussed in the past at several occasions. It appears to have been impossible, or at least very difficult/cumbersome in the past.
Will ExpressionEngine 2.0 bring any kind of change to this? I’m sourcing software for a new project, and the first item of importance is: vBulletin has got to work. If EE won’t work with it (unified login, etc.), then I it is not an option. You might discuss this ‘til you’re blue (“what features do you need”, etc..) but I simply think EE forum module is not as capable (i.e. not as configurable, extendable, lacks 3rd party extensions) and it is also not very aggressively developed. I think it would behoove EllisLab to provide integration for vBulletin; concentrate making a good CMS with optional basic forum module, or an optional vB bridge module.
In the old discussions on the topic it was pointed out that if one starts building/maintaining bridges, were will it end? Just limit it to vBulletin! There are some other forum software out there, but few as widely used.
It is my belief that by preventing vB integration EE negatively impacts its sales. It is quite apparent that EllisLab resources are stretched thin. That is, for example, why Kohana exists - people got tired of waiting for updates to CI, so they forked it.