My thoughts.
There is currently not much interest in a vB to UBB script. Why? Because the majority of users don't have this avenue ... most users are migrating from Infopop products to vBulletin and not vice-versa.
If Infopop saw people moving from vBulletin to UBB, then they'll be more inclined to create such a script but until this happens, you're right - they're probably not going to create such a script.
qasic