Create a plugin at hook: "forumhome_complete"
PHP Code:
// Hook: forumhome_complete
vB_Template::preRegister('FORUMHOME',array('notifications_total' => $notifications_total));
This registers the variable "notifications_total" on the FORUMHOME page, which is available.
Here is a great guide by @Cellarius on the subject.
https://vborg.vbsupport.ru/showthread.php?t=228078