also, check vixen that you're using the Enttec DMX USB Pro plugin, not the Open DMX.
while in Vixen, make sure the you're NOT using the DMX addin (Menu Add-ins... make sure there is not a check mark beside it.)
also, just to be safe, make sure the dongle is plugged directly into the computer, instead of into a hub at first.
and as strange as this sounds, try a different USB cable. try one from somthing you know that works well, like a printer.