Create short URL with auto 301 redirect

What you’re trying to do, can be achived by simply using the parse_request hook; no need for the custom rewrite rule and tag.

1. So remove this (and you should flush the rewrite rules — just visit the permalink settings page):

   add_action( 'init', 'vacature_rewrite_rule' );
   function vacature_rewrite_rule() {
       ...
   }
   

2. Then use this modified function which performs the redirection:

   add_action( 'parse_request', 'wpd_catch_vacature_requests' );
   function wpd_catch_vacature_requests( $query ) {
       if( ! is_admin() && preg_match( '#^redirect/([\w\-]+)$#', $query->request, $matches ) ){
           $posts = get_posts(
               array(
                   'post_type'  => 'vacatures',
                   'meta_key'   => 'vacature_id',
                   'meta_value' => $matches[1],
                   'fields'     => 'ids'
               )
           );
   
           // Redirect to the post.
           if( ! empty( $posts ) ){ // a valid post found
               wp_redirect( get_permalink( $posts[0] ) );
           // Or otherwise, the homepage. Or you can remove this and a 404 page would be shown.
           } else {
               wp_redirect( home_url() );
           }
           exit;
       }
   }