Redirects the login page for a mapped domain to the non-mapped domain.

wildcard_admin_redirect.php

function wildcard_admin_redirect($location, $status = 302) {
    $url = parse_url($location);
    if ($url['scheme'] === 'https' &&
        (preg_match("/\/wp-[login|admin|register]/", $url['path']) > 0)) {
        $orig = preg_replace("/^https?:\/\//", "", get_original_url($url['host']));
        if ($orig && $orig !== $url['host']) {
            $location = 'https://'.$orig.$url[path]; 
        }
    }
    return $location;
}
add_filter('wp_redirect', 'wildcard_admin_redirect');