Add this code in your child theme functions.php :
add_action('template_redirect','wplms_Restrict_non_logged_user_free_unit');
function wplms_Restrict_non_logged_user_free_unit(){
if(!is_singular('unit'))
return;
$free = get_post_meta(get_the_ID(),'vibe_free',true);
if(vibe_validate($free) && !is_user_logged_in()){
wp_die('Access not allowed');
}
}