Video This theme has a special embedding script that automatically resize the youtube video and makes it responsive. To use it, simply link to the video instead of putting the url. If you wish to use the native WordPress embedding method, it is still working!!
Posted as per the instructions in the Codex.
[wpvideo tFnqC9XQ w=680] VideoPress, especially as a video post format, usually provides some unique styling issues. You will need to install Jetpack or Slim Jetpack plugin to turn the shortcode into a viewable video.