There aren't free modifications available that can do anything near everything vBSEO does though. Even if there are, you'd have to install many modifications that would be a huge pain to upgrade and no gaurentee that they'll work in future versions.
You know, I have looked at the source code of two vBSEO-enabled forums, one of which is their own. I see nothing in that source to demonstrate special SEO-style meta tags or keywords or other content that would enhance SEO placement. Nothing. In fact, since I'm an Admin on an SEO-enabled forum (not my own), I tested the voluminous settings, and found that some don't appear to even function. Snake oil!
I use four mods for SEO on http://forum.theparacast.com: Automatic Thread Tagger, TfSEO (URL rewrites), vBSEO :: Sitemap Generator (it works and it's free -- but again a couple of the options are simply placeholders and non-functional) and ZH - Meta Tags. Original installation may have taken a few more steps, but the upgrades are easy, and the end results are, I think, equal or superior. And they're free!