Is it possible to make vlc play audio to only one of the speakers left/right from commandline ?
If VLC Audio channels is not performing well the most you can do is open a ticket in VLC bug system.
If you can't wait until the issue is fixed just redirect the audio output to STDOUT and use a tool like SOX to do the work for you.
Selecting a Particular Channel with sox:
sox stereo.wav -c 1 mono.wav avg -l
of course sox can work on streams and it's not restricted to files.
BTW, sox comes with play and record utilities that can do the actual playback. see the examples here