I run iTunes all day while I work, and have built in the muscle memory of pressing F8 to pause and play music.
Occasionally I work with Keynote as well. When music is playing and I press F8 to pause it while Keynote is the frontmost application, Keynote starts playing the presentation.
I want all presses of F8 to pause/play music in iTunes and never anything else. I don't use QuickTime, don't need it for VLC or Keynote.
I've seen elsewhere that users say this isn't something that can be changed, and I've submitted a feature request to Apple to change that. Perhaps the gurus here will know something different?