You can set a Video background to any Visual Composer row. Click on the Edit (pencil) icon at the upper right of the row.
This will open the Row settings dialog. Click on the Video Background tab at the top of the dialog. Choose to enable the video background and then add your video URLs to the appropriate fields. You will also want to set a fallback image for mobile devices. You can also set a overlay color.
Preparing a Video
First, you will need to create a video if you don’t have one ready. Save the video in .mp4 format. Then use a free video convertor like this one (http://www.mirovideoconverter.com/) to convert it to .webm.