Youtube has changed some things on their side with embed videos.
I have found a fix for this (you need to edit 1 file from the plugin)
edit the file "vipers-video-quicktags.php"
search for > if ( $atts['hd'] )
$hd = '&hd=1';
change/edit into > if ( $atts['hd'] )
$hd = '&hd=1&vq=hd720';
now all youtube vids are played in 720p
don't forget to select "Enable HD video" in the plugins settings. I tested all video's on my site and its working great!