For fast & easy access to your downloads once they have 100% finished downloading, go to your LW Library window, select Saved Files folder & press the Explore button. Sample image here: Using Library Explore button to access downld folder (click on link) & the folder will open up for you. For videos, all you should need to do is double-click them.

To preview incomplete files, they will only preview as much or less than you have downloaded. So if you have downloaded 10% of a 120 minute video, then you will only be able to view 8-12 mins worth. Files that download don't necessarily download from the beginning of the video to the end, but may download part of the beginning, part in middle part near the end, etc. *** -> I would recommend instead of using the Preview button (which creates preview files in your incomplete folder & wastes space on your drive), that instead, you go to your Library window, select Incomplete folder, list the files in alphabetical order by name (press name tab at top of column) & find the video you are wanting to preview. Once found, select it & press the Launch button. This will save you from cleaning out junk preview files from your incomplete folder at a later time. (Preview files are named like this: eg: PREVIEW-0170000-title.mpg)

To set VLC as your default video player, see this link for instructions Setting Default video app