vBSEO allows admins to customize all vB URLs. This is nice to have, but regarding SEO there's no difference between default XF and customized vBSEO URLs.
vBSEO also has some other less important on-page SEO features for improving vB code and content. There isn't such a big need for these features in XF, since it already has modern SEO optimized code and content. That's not to say XF couldn't benefit from a XenSEO, but I'd say it wouldn't be much. That's probably one of the reasons vBSEO isn't interested very much in developing such a product. Hope this helps.