Templates get compiled when you save into actual PHP code, and evaluated before it gets to global_shutdown.
global_start (or vb4 equivalent)
PHP Code:
echo $vbulletin->templatecache['TEMPLATE_NAME']; exit;
See what that prints out, and you'll have to do str_replace() on that variable, rather than $output.
PHP Code:
$vbulletin->templatecache['postbit'] = str_replace(
$find,
$replace,
$vbulletin->templatecache['postbit']
);
--------------- Added [DATE]1309828392[/DATE] at [TIME]1309828392[/TIME] ---------------
Alternatively, you could probably do it all in PHP.
Once you've rendered it once, set $post['signature'] = '';