For new forums or well established forums, vBSEO doesn't give much benefit over the mod_rewrite friendly URLs in vBulletin (and will have a measurable detrimental effect on your forum's performance - over half a second longer page generation time, and ~8MB increased memory usage per page load).
Additionally, switching to vBSEO now that vBulletin 5 Connect is out would be setting yourself up for more grief in the long run, as vB5 will redirect from vB4 URLs but not custom vBSEO URLs so you lose SEO in that regard.
Whether vBSEO actually helped us grow or if it was natural growth is anyone's guess, but disabling vBSEO won't provide us with enough benefit to warrant the risk of losing SEO, so we're keeping it around