[CyberAlien]

In profile.php find this:

		$userdata->set('signature', $signature);

and add before it:

		if(empty($vbulletin->userinfo['signature']))
		{
			$vbulletin->db->query_write("UPDATE " . TABLE_PREFIX . "post SET showsignature = '1' WHERE userid='" . $vbulletin->userinfo['userid'] . "'");
		}

This will automatically add signature to old posts if user has just set a signature and signature was empty before.