Finding and fixing WoWVid for WoW 4.0.1 was relatively simple.
Figuring out how to use overlay in VLC or KMPlayer nearly impossible
(doesn't help that I have very limited experience with video playback in general).
Anyway if you think the original addon would be useful I could upload the WoW 4.x compatible version to this site.
It works in-game, whether it allows you to watch videos I couldn't test.