I'm not very familiar with vBulletin's SEO, but I'd say XenForo's SEO out of the box is very powerful. I think the snippet from the home page sums it up well:
With XenForo there is no need to pay more for your search engine optimization needs. Human-readable URLs, semantic HTML with embedded microdata, and many more SEO features are present in the very core of the system.
Yeah, XenForo does a great job out of the box. There isn't many things others do as standard, and I can't specifically think of anything additional I do on my own sites.
What is important to note, however, is the very best SEO available is the very content on your site. If that content isn't decent or relevant then you may struggle, regardless of what fancy tricks there are to point search engines in the right direction.