I've fixed my instance of this mod; here's how (worked on vB 4.0.5).
> Go to vB admincp
> Navigate to 'Plugins and Products' -> 'Plugin Manager'
> Under 'Product : SEOforums.org SEO Link Replacement' click the 'Replacer main' entry
> In the 'Plugin PHP Code' box, change all occurrences of '$vbulletin' with '$this->registry' (4 occurrences in all)
Hope this helps,