I create a new page with “HomePage” template.
I add a new row with visual composer
Then, I click in the pencil icon (edit this row) and set “Use Video Background”, in Section Layout I set as “Default”.
Done this, it show me a blank screen not fullscreen like this:
Please use our custom layout for row element to make it show as the demo.
You can import demo pages then edit them as you want.
There is new update for youtube video background support, please download then update the theme: http://awesomescreenshot.com/0bf5fsje5d