Raw isn't showing in 'header'

[Yellow Slider]

When im adding the template to a raw variable, and showing it on the template 'vbcms_page', it works.
But when im adding this to the 'header' template, id doens't.
Here is the plugin:

$abcHTML = vB_Template::create('DK')->render();

vB_Template::preRegister('header', array('var' => $abcHTML));
vB_Template::preRegister('vbcms_page', array('var' => $abcHTML));

As you see, the template named 'DK'.
I think the HOOK location isn't good. the location is: process_template_complete.
but if this is really the problem, why? and i will be happy for a little explain about 'HOOK'.