the_post_thumbnail unless video id is added

I think all you need is this:

<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
  if (get_field('channel') == 'Youtube') {
    echo get_field('youtube_video_id');
  } elseif (get_field('channel') == 'Vimeo') {
    echo get_field('vimeo_video_id');
  } elseif ( has_post_thumbnail()) {
    the_post_thumbnail();
  }

That should use a Youtube video, then a Vimeo video, then the thumbnail, and do nothing otherwise.