Replace slider image with an MP4

On the Church of Zero website, I have an animated GIF on the front page. It is a 7MB image, way bigger than I want. The original was an MP4 just over 2MB. I have Space2.mp4 and Space2.webm uploaded, but cannot get the video to display.

I am using the “Video Background” plugin to support this, but can’t find the right place to slot it in. Here is the way it currently is set up in Styling->Custom CSS:
{[vidbg container="header_container home.flexslider .slides img" mp4="http://www.churchofzero.com/wp-content/uploads/Space2.mp4" webm="http://www.churchofzero.com/wp-content/uploads/Space2.webm" poster="#" loop="true" overlay="false" overlay_color="#000" overlay_alpha="0.3" muted="true"]}

I assume my container is incorrect, have tried several options.

Thanks for any help.