[kh99]

The problem is that in vb3, templates were stored as if they were a php double quoted string without the quotes, so you could just replace part of one cached template with another entire template. In vb4, they're stored as php code that when eval()'d sets the $final_rendered variable to the html string to be output. So if you think about that, just inserting another template doesn't work - you end up with invalid php code.

In any case, you could try this:

$myput = '<!-- message area -->';
$replace = '\'; $final_rendered .= \'' . $myinput . substr(fetch_template('new_posting_newthread'), 19) . ' $final_rendered .= \'';
$vbulletin->templatecache['newthread'] = str_replace($myput,$replace,$vbulletin->templatecache['newthread']);

Edit: what badshah posted above is obviously simpler and will work if you haven't used any variables from the newthread template.