How to integrate vBulletin into WordPress (theme)

I have been wondering this for a while now. I am not sure if it is a simple plugin for the forum/wordpress or do they have to make the same header in vBulletin again.

One example:
http://www.organicthemes.com/support/
The forum is part of a site which also includes a blog. It is not part of the WordPress

http://www.levelunlocked.net/forum/
The forum might not be vB, but it is exactly what I am talking about, having a forum inside WordPress. It's not really inside, but they share headers which is what I am looking for.

http://androidforums.com/
Another example. Click the home button to see the blog. Not sure if it is WordPress, but this is the sort of work design I mean.

http://forums.androidcentral.com/
Remove the "forums." to see the blog. Again not sure if that is Wordpress but that is the thing I mean.

How can this be done exactly if I want to have the same header for my blog and forum so it all seems like one site?