try the iframe shortcode plugin
http://wordpress.org/extend/plugins/iframe/
Embed iframe using shortcode [iframe
src=”http://player.vimeo.com/video/819138″ width=”100%” height=”480″]
ps: you can also use the default oembed shortcode:
...
it supports YouTube, Vimeo, DailyMotion, Flickr, Twitter, … and more.
Example:
Screenshot:
See more here: