In order to add video background like this page for example Sermons page, please refer the steps below:

Step 1:

Create page and insert "column" Vamtam drag and drop element.

Step 2:

Click on the pencil icon of the column in the Vamtam Drag and Drop editor:

Step 3: 

Set the video background in the column's background options:

Background Video - Insert self-hosted video.  Allowed formats are included in this article

Please note that the "Video Background" option will be visible only in case that the "Background Parallax" option is disabled.

Once you've inserted the Background video, you can re-activate Background parallax option:

Step 4:

If you need the video in the header where the slider is located, go to  the page's local Vamtam options panel and turn off the "Show Page Title Area" option: Also make sure the slider is turned off for that page.

Step 5:

At last please hit the Save element and next Update to save all changes.