There is a great plugin for wordpress called Tubepress. This will basically take any collection of videos on Youtube or Vimeo and will embed them as a gallery in your wordpress site. This blog will show you how to change the default vimeo thumbnail from the '100' px version to any of the others.
I will update this tutorial later, but for now here is a quick quide. All you have to do is navigate to teh file shown below, and open it up for editing.
plugin\tubepress\sys\classes\org\tubepress\impl\factory\commands\VimeoFactoryCommand.class.php
Now scroll to the bottom and find the function below and change the $raw[0] to a $raw[1] or $raw[2]. You will see the quality of your thumbnails upgrade instantly.
protected function _getThumbnailUrlsArray($index)
{
$raw = self::_gatherArrayOfContent($this->_videoArray[$index], 'thumbnails', 'thumbnail');
return array($raw[0]);
}