For including videos in your posts, just copy and past the embed iframe code, which is generated by the video platform, into the post content. The theme will take care of the responsiveness.

Example:

<iframe src="https://player.vimeo.com/video/146176651" width="500" height="281" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>
<p><a href="https://vimeo.com/146176651">Gallery Stack Demo</a> from <a href="https://vimeo.com/user44621138">Jannis Rondorf</a> on <a href="https://vimeo.com">Vimeo</a>.</p>

Will result in:

Gallery Stack Demo from Jannis Rondorf on Vimeo.