If you are trying to make a tab that links to an already established page such as "content.php?139-blog", I found a solution:
First modify your plugin to include the end piece of your url in this case "/content.php?139-blog". This will enable your tab to be highlighted.
PHP Code:
if (THIS_SCRIPT == 'vbcms' && $_SERVER['REQUEST_URI'] == '/content.php?139-blog')
Next you want to make it so the new tab is the only selected so go to plugin manager and edit "Navbar: Insert CMS Navbar Entry".
Change this:
PHP Code:
if (defined('CMS_SCRIPT') AND class_exists('vBCms_Navbar'))
To This (except fill in your page name):
PHP Code:
if (defined('CMS_SCRIPT') AND class_exists('vBCms_Navbar') AND $_SERVER['REQUEST_URI'] != '/content.php?139-blog')
Viola!