I moved away from vBulletin because I was disappointed with the 4.0 product I had purchased a license for in the pre-sale (circa. Oct/Nov 2009).
I move the Invision Power Services IP.Board. This was before I'd even learned about XF and at the same time I was moving away (April 2010) lots of other people were too (there is printable evidence in the IPS forums to indicate this as well).
So ... I'd be quite happy to testify on behalf of XF to demonstrate that it wasn't XF that turned me away from the vBulletin software platform, rather it was the actions (or inaction) of VBSI and their poorly constructed, implemented, and bug-filled product which was vB 4.0 (and subsequent poor quality updates).
Cheers,
Shaun