I've been wondering about this for a while. The current vbulletin design is a whole lot better than before, and I like how the pages are surrounded by the gray sides.
However, for some reason, the header doesn't have that gray line around it, it just suddenly stops at the top. That makes it seem.. unfinished. It's just my opinion of course, but wouldn't it look better like this?
Now you have the whole site surrounded by the gray lines, which makes it look... better.