vBSEO works because vBulletin needs it. I used it for several years and it did an amazing job for us.
Xenforo doesn't need a separate SEO software; but if those folks ever release it for XF - I might consider buying one.
It's useful if a customized URL structure is required. That however is a more personal preference than a SEO issue. But there is a small market for such an add-on, as multiple people have asked about this.
I'm also on the same boat! I've a vB 3.8+vBSEO board which I intend to port to XF at some time in the future. But I really like the custom URL structure of my board (via VBSEO) and would love to continue using it if I can. i.e. have the vBSEOed urls work properly with XF without any redirects.
This should cut down any re-indexing issues or potential (hopefully not permanent) drop in SERPs. So far, I haven't found any solutions for doing that with XF.