Do you mean, you want to disable VLC launching when a video file has finished downloading or do you mean you dont want the video to start playing when you open it. If its the first option you will need to disable auto launch in the download manager. If its the second option:-
Open VLC and open 'Preferences'
Select 'All'
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Then navigate to 'Interface' - 'Main Interface' - 'MacOSX'
then de-select the 'Auto-Playback' option
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Hope this helps.