Hi Rick,
Someone clicking the Play button on an embedded video is not a trackable action by Marketo. I beleive it's possible to use JavaScript to create an onclick javascript event that can track someone clicking the play button within an iFramed vimeo video. Though, I've never been able to track down the correct snippet of code to do this...
As a workaround, you can replace the iFrame with a Screenshot of the Video, and link directly to the vimeo video. This way you have a Marketo trackable event, of 'Clicks Link' which you could score off of.
For an improved user experience, I'd recommend linking to another Marketo page that you've set as a Lightbox Overlay, which contains only Vimeo video set to Autoplay. You can use this as a starting point:
<a href="PAGE" class="html5lightbox" data-width="800" data-height="450" ><img src="SCREENSHOT" width="900" height="500"></a>