Nicely put. I'll add a twist to what gio~logist said as well. I'll come from a designer's perspective. It's quite early in the morning (3:53am) so I hope this post will translate well.
vBulletin comes equipt with the necessary functions/features to run a successful forum. It takes the effort of the staff, members, content and the overall feel of the community that determines if the site/forum reaches its potential. Part of the feel, however, has to do with the design/layout.
I've had many users register and become members because of the site/forum design, which changes routinely over time. The majority of the compliments also involves the design. At least for a gaming site, changing the layout, modifying the colors and adding graphics do make a difference that sets the competition apart.
Additional modifications are there to enhance the users' experiences and functions of the site/forum. To be true, the design is also an enhancement. Neither of the two will make the site/forum successful; they're basically the icing to an already tasty cake. If the cake is bland to begin with, there's not much a modification or design can do.