Or to be more precise, a window can’t go fullscreen on the second display at all using Change Fullscreen Status . It will go fullscreen on the second desktop of the main display, not on the external display.
The List Screens node lists the external screen and its resolution correctly, but the Change Fullscreen Status does not seem to work as expected.
In Vuo 0.9.0, we stopped using the new (Mac OS 10.7+) fullscreen behavior, so now you can make multiple windows fullscreen on 10.7 and 10.8. (This already worked on 10.9 and 10.10, if you had the new “Displays have separate Spaces” setting enabled.)