There's currently no converter for Xenforo to SMF (any version).
Xenforo is reasonably common now, but does cost $ (as do most of the mods, themes) and the expense appears to be gradually increasing for people using that forum platform. So there are people looking for alternatives.
With SMF 2.1 RC4 having been released and the final version being worked on; now would seem an opportune time for a Xenforo to SMF 2.1 converter to be created.
I saw elsewhere that someone ( AllanD ? ) had done a conversion by Xenforo --> myBB --> SMF which apparently worked.
I'd imagine that would require a fairly detailed knowledge of Xenforo, myBB, and SMF. And that certainly doesn't describe me!
Given that upgrade path did evidently work, indicates to me that a converter direct from Xenforo to SMF should be possible. (and the obvious candidate now would be to SMF 2.1
Perhaps someone with the appropriate technical and coding know-how would like to take this on?
(No I can't afford to pay for this, and I have no immediate need, but it's possible I may need it in future.)