Ok, the code of this plugin is not the best but I did not check it before.
I created my own addon in 2018... that code work!
Replace everything in your file with:
PHP Code:
<?php if (!defined('VB_ENTRY')) die('Access denied.');
error_reporting(E_ALL & ~E_NOTICE);
vB_Cache::resetCache();
vB::getDatastore()->resetCache();
log_cron_action('Cache cleaned!', $nextitem, 1);
?>
That should work!