When blog post has embedded video it usually uses iframe which is preferred way because this support non flash browsers like iPad.

Code is something like this:

<iframe src="http://player.vimeo.com/video/36577369?title=0&byline=0&portrait=0&color=80ceff" width="500" height="281" frameborder="0" webkitallowfullscreen="webkitAllowFullScreen" mozallowfullscreen="mozallowfullscreen" allowfullscreen="allowFullScreen"></iframe>

During RSS parsing it's converted to self enclosing tag <iframe ... /> which doesn't work.

Example to check: http://planet.jboss.org/post/small_fosdem_2012_review