Looking into WordPress Core vimeo is registered like so:
'#http://(www\.)?vimeo\.com/.*#i' => array( 'http://www.vimeo.com/api/oembed.{format}', true ),
But looking at vimeo API for oEmbed they show their URL without the www.
so what you need to do is register vimeo oEmbed correctly:
add_action( 'init', 'add_vimeo_oembed_correctly' );
function add_vimeo_oembed_correctly()
{
wp_oembed_add_provider(
'#http://(www\.)?vimeo\.com/.*#i',
'http://vimeo.com/api/oembed.{format}',
true
);
}
You can place this in your theme functions file or place in a plugin.
You may need to revisit posts that have Vimeo embeds and save again after putting this fix in.