Many people have suggested they can't migrate to xenforo until search within a thread is available.
Albeit not as elegant as a xenforo core solution ... I think it still can be helpful ...
Idea: use google's site: searching but use the entire thread URL (not just xenforo.com).
This seems like it would be a very easy addon to make.
this was one of my favorite features of vb, and i miss it a lot in XF!
Even more critical for when add-on support threads here get into the scores of pages