Unassign what exactly? It doesn't matter what space is assigned to. It doesn't get trapped by VLC because Accessibility (full keyboard access) invokes the action instead. There's no good reason to have the volume down button be the default selected control. I don't have the source to see what is assigned as FirstResponder, but it ought to be the play/pause button, and almost surely isn't, if one was even assigned at all. I am not a developer though, so please correct me if I'm wrong.
Take a look at this screenshot. You can see the volume down button selected. This is on launch, with no media having been loaded. You can eventually tab to play/pause (after cycling through everything else).
Again, I can't see any good reason to have it this way, and it seems pretty clear that this would be a low risk fix.
Also, FWIW, you'll find that most Mac power users have full keyboard access enabled, as keyboard navigation on OS X was fairly well thought-out...it's quite efficient.