Firstly, you need to find some vBulletin forums. Head on over to Google and enter the following:
"Powered by vBulletin"
This will bring up a list of good ranking forums. Now I know I've mentioned this a million times before, but I can't stress this enough: RELEVANCY! So let's suppose you're niche is used cars. So we could add the following to narrow our search down:
"Powered by vBulletin" + "used cars"
We should have a shorter but more specific list now. Our first hit is www.carforums.net.
Now what we're looking for here are threads within this forum that have a higher than usual PageRank. To do this you'll need to first install a FireFox add on like SEOQuake or something similar. SEOQuake is great because it offers more than just PageRank.
Now with this installed we can quickly see PageRank alongside our SERPs. So what we're going to do is search within the forum for threads with a high PR and focus on those.
In Google, search through a site by using the "site:" modifier like so:
As we can see, the site has a PR of 3 and if we scoot through the SERPs we can see that the majority of threads have not been assigned PR, but we can also see lots with a PR of 2. Join the forums and comment on these particular threads. Remember to keep it on topic and make it a useful addition to the forum, please don't spam. It's pointless anyway as you'll most likely get removed if you do.
Repeat the process for all the forums you can find and all the threads you can add something to. If you've nothing to say, don't bother saying it. Only comment if you can genuinely add to the topic.
I've started working on the SEO plan for James Inside Out Health Magazine website, I'll begin posting tomorrow!
In the meantime, Happy SEOing.