How do I adjust which apps are controlled by the play/pause button on my MacBook Pro?

I’m currently watching a movie using VLC, and occasionally I pause the movie to take some notes. The only problem is every time I hit the play/pause button it opens iTunes and plays/pauses videos/music in iTunes. I’ve resorted to turning the volume in iTunes all the way down, but for convenience and battery life, it would be a lot better if I only had one video player playing at a time. Does anyone know how I can get the play/pause button to only affect apps that are already open?