Depends on the plugin. I mean, vB SEO was pretty useful back when vBulletin 3 was the best software around. And I like the WordPress SEO plugins that exist.
I've been using vBSEO for many years and have noticed massive improvements when using it. The only thing which concerns me at the moment with vBSEO is that it doesn't seem to be being developed and the owners / support staff are non existent.