Tyler and I had a lot to do with that decision as we handle a lot of the infrastructure for the website. The main reason for using vBulletin was ease of installation, the ability to migrate data from the old board, which was more or less a highly hacked phpbb board, the support offered for vbulletin, the community that uses vbulletin, and the hacks available. Honestly a lot of it came down to the support as I have always handled the technical side of the board to some degree, but we were left hanging with the last board and had a board that had been severely modified and no one wanted to touch it with a 10 foot pole. We couldn't upgrade. We couldn't move servers. We couldn't even get a good backup. vBulletin provided us a path that we couldn't find anywhere else.
I don't want to make it sound as if that was the only reason. I believe vBulletin is a great product. They are on the ball with releasing patches for security and seem to genuinely be interested in what the community needs. For TDH's purposes, we didn't want a lot of bells and whistles. We simply wanted a very stable core for everyone to be able to post to. I think vB has fit that bill very well.
I looked at Invision Board and got involved when the RPF moved to it. Personally, I didn't care for it as much as vB but it seems to have fit their needs fairly well.
