Set up a fullscreen video background (video)
Choose Messy template from the drop down in Settings / Template
Add sections to your site in Sections / Create new section
Open a Vimeo video link and click "Share" button
Go for "Show options" button
Tick "Autoplay this video" and "Loop this video" to launch video automatically for your site and make it loop
Copy the embed code above
Paste the embed code into your site in Sections / Details / Background video (embed)
Your video can fill all the window or keep its ratio. To make the choice, go Sections / Details / Background video ratio. By both choices your video will be responsive and will continue its playback.
Go to you berta site and check out the result!