If the custom field contains just the URL you can achieve this using wp_oembed_get
$video_url="https://vimeo.com/75791532";
$video = wp_oembed_get( $video_url );
echo $video;
If the custom field contains other content as well something like this should work.
$content="<p>Check out hte latest vid!</p>
https://vimeo.com/75791532";
$content = apply_filters('the_content', $content);
echo $content;
the_content filter automatically applies the oembed filter.