It's currently not possible to force VLC from stereo to mono mode, or from stereo to reverse stereo or dolby mode. It's one of the few options in VLC that can not yet be forced from Preferences or the commandline.
Status: Todo
+Task
+Difficulty: Hard
+Platform: any
+Urgency: Wishlist
+Description: Java wrapping/bindings
+Allow java apps to use VLC perhaps? Several people have suggested something like this.
+Status: Todo
+
Task
Difficulty: Easy
Platform: win32/macosx