Video embed not resizing on mobile

Hello, I’ve a problem when I embed a video in a new post. When I see the post from a mobile device, I see only half width of the video, as it’s in an iframe and it has a fixed width, bigger than the one of the mobile device. Youtube works fine and adapts his width (I only paste the link on a blank line and wordpress does everything), but Vine and the default player of wordpress have this problem.

What can I do?

Thanks!