[kh99]

Use hook parse_templates - process_templates_complete is too late because the header template has already been rendered.

Try this code:

$menuUrl = $vbulletin->options['bburl'] . "/menu/index.html"; 
$menuContent = file_get_contents($menuUrl); 

vB_Template::preRegister('header', array('menuContent' => $menuContent));

And use {vb:raw menuContent} in the header template.