Hi,
in the preferences one can select a full screen playback display if there is more than one monitor connected to the computer. however the selection of the display is very unreliable!
it seems to be pure luck that vlc plays back on the screen i selected. on some machines i can not get it to play on the selected display at all!!
sometimes it works when i change another option in the settings along with the full screen output display.
i've been having this problem on mac and pc (xp64, win 7 x64) computers and i have been testing this for more than two years now, but no update has sorted this out!
this is my number one bug fix request (i hope this is the right place for it).