You should parse Vimeo's API's response. There is no way to it with URL calls (like dailymotion or youtube).
Here is my PHP solution:
/**
* Gets a vimeo thumbnail url
* @param mixed $id A vimeo id (ie. 1185346)
* @return thumbnail's url
*/
function getVimeoThumb($id) {
$data = file_get_contents("http://vimeo.com/api/v2/video/$id.json");
$data = json_decode($data);
return $data[0]->thumbnail_medium;
}