When you want to embed video from Youtube or Vimeo in WordPress article you just enter URL of video into text.
There is one very useful plugin Raw HTML. This plugins allows you to include HTML content in post.
I was facing strange issue. When I turned on Raw HTML plugin then all embed videos stopped working. I was searching for cause of the problem. It seems that there is problem with priority of filter "wpautop" in Raw HTML code.
Original source from Raw HTML contains following code - raw_html.php - line: 139