I know a forum owner who has concentrated soley on new signups and member numbers, and as a result they have a huge membership but no posters, no new threads.
I think that the thing with forums is about the posts and the quality of the posts & the discussion and debate. Some of the longest running threads on UKBF are controversial. Forums that are full of "hi this is me", and "use me, use me, use me!" posts are very boring, and don't encourage visitors to come back. (Not saying that there are a lot of those here, but it is a bit of a forum killer, and when you only get lots of new sign ups this tends to take over the forum)
FBF is an interesting forum, a very small membership but a very active core, you can be pretty much guaranteed that everyday there will at least a two very current threads not solely business orientated, also picking up on news and politics, which get a bit of debate going. This makes it worth "looking in" every day.
One really cheap way of online marketing may be simply just to get together a group of people, who aren't afraid of a bit of discussion and debate, to take on some current and maybe contraversial business and maybe non business topics, and make sure that each of posts new threads and answers every day.
I think the thing with forums is that people are much more likely to respond/answer a thread than to start a new thread. So maybe for a while, you give them something to talk about.
You already use Vbulletin, which as far as I can see is one of the most SEO'd up forum softwares. If you implement the tagging, and your above posters use the tagging on the current and contraversial posts, then i think that should improve the online side of things with very little outlay.
Hope this helps.
Kind regards
Sandra