Your issue is that VLC on the Pi 4 renders video in a separate window by default, ignoring the MagicMirror module position. To fix it, set pixel dimensions instead of percentages and add localPlayerArgs: ‘–no-video-on-top --width=640 --height=360’ so VLC respects the module container.
Example:
position: "top_center",
moduleWidth: 640,
moduleHeight: 360,
localPlayer: 'vlc',
localPlayerArgs: '--no-video-on-top --width=640 --height=360'