In WordPress, if a featured image isn't set. check for a custom field called 'video' and if there's a url in there from either Vimeo or YouTube (including youtu.be shortened links) nab the thumbnail.

get_video_image.php

<?php 

// Check for a featured image with get_the_image by Justin Tadlcok http://themehybrid.com
// if one is not found check for a custom field called video
// if there is a custom field called video and it has an address from Vimeo or YouTube, grab the thumbnail
?>
<?php 	$thid = get_the_ID();
		
		$vim_prefix = 'http://vimeo.com/';
		$yt_prefix = 'http://www.youtube.com/watch?v=';
		$yts_prefix = 'http://youtu.be/';
		
		$str = get_post_meta( $thid, 'video', true );

			if (substr($str, 0, strlen($vim_prefix)) == $vim_prefix) {
   		
				$str = substr($str, strlen($vim_prefix), strlen($str));
				$imgid = $str;
    			$hash = unserialize(file_get_contents("http://vimeo.com/api/v2/video/$imgid.php"));

    			$default_image = $hash[0]['thumbnail_large']; 
    				
    		} 

    		elseif(substr($str, 0, strlen($yt_prefix)) == $yt_prefix) {
	    		
    			$str = substr($str, strlen($yt_prefix), strlen($str));
    			$imgid = $str;

    			$default_image = 'http://img.youtube.com/vi/' . $str . '/0.jpg';
    			
    		} 

    		elseif(substr($str, 0, strlen($yts_prefix)) == $yts_prefix) {
	    		
    			$str = substr($str, strlen($yts_prefix), strlen($str));
    			$imgid = $str;

    			$default_image = 'http://img.youtube.com/vi/' . $str . '/mqdefault.jpg';
    			
    		}

			if(function_exists('get_the_image')){
				get_the_image(array('default_image'=>"$default_image"));
			}

?>