You'll only be able to launch & preview
within LW using VLC if you're using one of the LW Beta versions 4.13.0 & onwards. Latest Beta is 4.13.10.
If you're using a stable release, you'll have to preview & launch from your incomplete & saved folders outside of LW. It's no drama.
To preview go to LW's library, click on Incomplete Files. Highlight the file & click Explore. Your incomplete folder will open, preview from there.
Same deal for launching, except that you'll go to Saved Files within the LW library then click Explore etc.