A discussion board can go through one of two distinct template lifecycles. Which one it follows depends a lot on the administrator of the forum and what they are trying to achieve with it.
This lifecycle is probably the more common of the two and probably accounts for many of the customized vBulletin boards you've come across.
This is where the administrator installs their copy of vBulletin and gets their forum going as quickly as possible. So, in order to minimize any delay between getting the discussion board installed and getting it working, they simply open up the forums for discussion using of the default style (or download and use another style that they fancy).
Then, with the board open and operational, ...