In function lb_vip_command_fn, the last "plen" param is redundancy. As follow

if (!unformat(line_input, "%U", unformat_ip46_prefix, &prefix, &plen, IP46_TYPE_ANY, &plen)) {
    error = clib_error_return (0, "invalid vip prefix: '%U'",
                               format_unformat_error, line_input);
    goto done;
  }