I have a JBL E40BT Bluetooth headset. It comes with media controls on the left ear cup, a play/pause button, and a volume up and down (previous and next track is handled by three and two fast presses of the play button, respectively). The volume up and down work on a system level, allowing me to control Windows volume.
The play/pause button however, doesn't. VLC, Media Player, even Groove.
Now I've tested this thing with my Android phone, where it works, and on my Mac, where the play/pause button opens up iTunes.
But for some reason, it won't work on Windows. I've checked services, and Human Interface Device Access is up and running, so I'm not sure what else I need to do.
On a side note, both my keyboard and my mouse comes with a play/pause, and they only work when I've got VLC / WMP on AND in-focus.