By default, all Vimeo videos will play inline on mobile browsers. Instead of entering fullscreen mode within the native device player, your video will play directly in the webpage where it’s embedded.
You can enter fullscreen mode by clicking the fullscreen button on the player.
How to prevent inline playback on mobile
If you prefer for your videos to play in fullscreen on mobile, you can add the ?playsinline=0 parameter to the end of the player URL in your embed code. This will force your video to load in fullscreen mode once the viewer clicks on play.
Example embed code:
<iframe src="https://player.vimeo.com/video/137805268?playsinline=0" width="640" height="360" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>