Although I do use vBSEO on one of my forums, love it, and attribute some of my SE traffic to my vBSEO implementation, I also have another vBulletin forum with much greater SEO success -- without vBSEO. This has proven to me that as far as vBulletin 3.x is concerned, you can make it to the top 3 SERPs without vBSEO (as I have done) in approximately one years' time if you do things right.
I agree with others in that XF already has great SEO functionality built in right out-of-the box and, as such, does not need a full-fledged system like vBSEO. I really do like a lot of the features that vBSEO has, but I'd much rather see the XF developers implement some of those popular features (e.g. Sitemap Generator, auto changing of link anchor text to title of destination page, etc.) in their own way, so that the built-in functionality is even further improved.
So in summary: XF is already great in terms of SEO, it could do with some additional features to add further functionality, but a vBSEO-like solution is not needed.