As an HTML Widget, this itself is an iframe. So you wouldn't be able to use code outside the iframe to control what is inside it.
This is for controlling the hype-native/html5 video element if you were to drag a video file onto the scene editor. So if you're using vimeo, this also won't work. If you can use regular video files, then this would be okay.
Otherwise, you might want to look at my post here, which shows how to stop vimeo videos: