I don't understand what you think this line does:
PHP Code:
$vbulletin->templatecache['header'] .= eval('testplate');
And you probably want to assign this to a variable:
PHP Code:
$templater->render();
PHP Code:
$myvar = $templater->render();
and then preregister it for use in a template:
PHP Code:
vB_Template::preRegister('TEMPLATENAME', array('myvar' => $myvar));