That's not hard, but as good as impossible. You'll have to compile VLC including all it's modules and plug-ins as a QuickTime Plugin, which is not possible ATM and would require huge amount of work. You should use simply VLC when you want to play multiple channels.
Btw. since version 4.6 of iTunes you are also able to multiple channels with it, but you have to deaktivate all sound filters, push the volume to the max, etc., etc. Please have a look at
this document for details.