Wont let me add to template var {vb:raw notifications_menubits}'s value in a plugin

[macmee]

{vb:raw notifications_menubits} is a variable in the header template. I am making a plugin and require adding additional content to this variable (it's the one that *should* stick content where I have drawn this red line ). I created a plugin and have tried both global_start and parse_template as hooks but the following never displays anything when I load my forum index:

$variabletest = 'llllllllllllllllll';
vB_Template::preRegister('header',array('notifications_menubits' => $variabletest));

All I'd like to accomplish here is being able to add text to the popup which comes up when you hit "notifications" at the top of the page, through a plugin I am writing. Any advice would be greatly appreciated!