VLC 3: Adding file to existing playlist (enqueue)
To add a file to VLC playlist from the command line (without starting a new instance) use the following syntax:

open -a /Applications/VLC.app/Contents/MacOS/VLC &lt;your_media&gt;


To prevent VLC from autoplaying the media you just added, go to Preferences > Show all > Interface > Main Interfaces > macosx and uncheck the box "Auto-playback of new items" below the Behavior subheading.