I see VLC as the swiss army knife that can play anything you throw at it, no matter how weird. I see mpv as a more streamlined, less abstract implementation that supports many fewer options, but can contain special-case code paths for hardware acceleration, for example. Hardware-accelerated playback on my Raspberry Pi 3 works out of the box with mpv but requires some special trick or doesn't work at all in vlc. I assume mpv's design allows it to contain a special-case code path when you happen to be playing something compatible with a Raspberry Pi's GPU on a Raspberry Pi.