vimeo-background-cover.html

<style>
body { margin: 0; }
.vimeo-container {
  padding: 0;
  width: 100%;
  height: 100vh;
  overflow: hidden;
  position: relative;
}
iframe {
  box-sizing: border-box;
  width: 177.77777778vh;
  height: 56.25vw;
  min-width: 100%;
  min-height: 100%;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}
</style>
<div class="vimeo-container">
  <iframe src="https://player.vimeo.com/video/[videoId]?api=1&background=1&autoplay=1&loop=1" allowfullscreen="allowfullscreen" frameborder="0"></iframe>
</div>