[tbworld]

I am not sure it is the right hook, I would have to look at it, but it should output.

session_start();
$PageCount = $_SESSION['PageCount']++;
vB_Template::PreRegister('header',array('PageCount' => $PageCount));

Make sure you are looking at the correct style.

For testing purposes make sure it is not nested into a conditional, thus blocking output.
{vb:raw PageCount}