I don't want to sound harsh but in the last year or so every single site i've known using ubb switched to vb. Since I don't watch the ubb forums closely I can't say this for sure but I can only presume it has something to do with users asking for database/php support and (for one reason or another) not receiving it. Of course, the fact that vb has an import script from ubb is another reason...
From Infopop's point of view wwwthreads could be just the thing it needes to become again the market leader, since it has some unique features -- including but not limited to database/php/threads -- not available in ubb. Of course, one can try to guess where the prices will end just by looking at the actual prices of ubb (which lacks above said features). It will also be interesting to see what will happen to ubb/it's price after the fall sale discount which ends on oct 14.
Only time will tell how and where is wwwthreads (or 'Threads' as it is refered now) going. I'm still willing to contribute translations / ideeas / fixes / hacks to Scream if that will be allowed and user input will be taken into account like it is now. I hope that this merger will prove to be one step forward -- if not, well... nothing is ireplaceable: neither the customers nor the software.