One of the most cost effective ways to drive more traffic to your vBulletin powered forum is to add and SEO hack to turn the default vBulletin url to a more search engine friendly url. You can do this manually but it takes an awful ling time and each time vBulletin is update you have to spend yet more time updating the hack.
The most cost effective solution is to use vBSEO . I use it for this forum and its been excellent at helping make the forum search engine friendly which in turn increases traffic.
vBSEO cost $149 but it will pay for itself in no time. You can buy vBSEO by clicking here.