For now it's not possible to use a thumbnail if you embed a youtube/vemeo player. A potential workaround would be to use the lightbox to display the videos or to add a featured image and a video to the portfolio items (eg user clicks on the thumbnail, the article will be loaded and displayed near the top of the portfolio grid and the user can use the slideshow button to switch from the featured image to the embedded video). Another workaround would be to change the thumbnail behaviour (Angular Options > Layout & Settings - search for the "Slideshow behavior on overview pages" option and select "Display only single image on overview pages and all slideshow images except the first one on single entries". This option will hide the first preview thumbnail and you'll see the featured video only. I'll report your feature request to Kriesi.