[kh99]

Try this: create a new plugin using hook parse_template and this code:

global $vbulletin;
$totalthreads = 0;
$totalposts = 0;
if (is_array($vbulletin->forumcache))
{
	foreach ($vbulletin->forumcache AS $forum)
	{
		$totalthreads += $forum['threadcount'];
		$totalposts += $forum['replycount'];
	}
}
$totalthreads = vb_number_format($totalthreads);
$totalposts = vb_number_format($totalposts);

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

Then you should be able to use {vb:raw totalposts} in the header (I know you didn't ask for totalthreads, but it was there so I thought I'd just leave it).