As you posted - these are the conditions that need to be true in order for the CMS tab to be highlighted:
Code:
if (defined('CMS_SCRIPT') AND class_exists('vBCms_Navbar'))
Most likely, unless you include the correct files in your custom page, you will not be able to meet the second part of the condition. So, it may be easiest if you modify that condition to add your own... add an OR in there:
Code:
if ((defined('CMS_SCRIPT') AND class_exists('vBCms_Navbar')) OR (your condition))