Caliente is correct... you have to manually extend the size of Vixen to both screens. One issue I think you may encounter... when you stop a sequence, the Adjustable Preview window returns back to it's original location on the first window. I though this was fixed on 2.5, but I haven't tested that version on my dual screens at work. I thought KC had the adjustable viewer window remember the size and location of it's last open state.
So after all the blabber above

if you can't get the window to open back up at the same location try version 2.5 of Vixen.