This guide will explain to you how to attract new users to your vBulletin forum (actually, it works on all forums, but it was made specifically for vBulletin)
#1) Always have a clean skin. Only have a dark, evil looking skin if you're running an 'underground' kind of forum. Like if you're doing warez and such. (which is I am completely against) If not, keep it a bright kind of style, with glossy icons and such (perhaps). This will not hurt the users eyes too much, and will allow them to find what they're looking for, easier.
#2) Never hide your sections from guests. By doing this, you'll loose so many people. They'll register, see your content, then just leave. It will take up much more bandwidth by closing the forums also (because of all the guests that will register then leave). Plus, search engines (Google, Yahoo...) will not be able to display any content if people search your site up.
#3) Only ever have a small amount of forums. When you get your forum, and only have a few members, don't add too many sections, or it will overwhelm your members. Plus, if you have sections without posts (if you have too many sections, then you most likely will not have many posts) then it makes your forum look more dead than it really is. Once you gain more members, that's the time to add new sections and categories.
#4) Don't have too many staff members. I've seen forums before, where over 80% of the active community, is staff members. This will kill your forum, as your staff get bored, and quit, plus no one will register because your site is too heavily moderated, and they'll feel weird all of the time.
#5) Always spend at least 10-20$ on advertising. That's a minimum. Go on other sites that are related to yours, and advertise your website there. But remember never to spam advertisements. That makes you look very immature, and a stupid little website that will do anything for a few members.
#6) Before advertising, make sure you have a bunch of posts, and a few members. When people see your advertisement, but see you have no posts or members, they will most definetely not join.
#7) Owners must not have over 30% of posts. Unless your board is new, or it's a company where the owner must post a lot. But never, ever have over 30% of posts, as it makes you look desperate for posts.
#8) Never go over board with modifications. No one likes a board with too many modifications to handle. Keep it down a little bit of modifications, not too many, not too little.