[calorie]

Look for the following in the vB showthread.php file:

// get ignored users

There is a chunk of code that creates the following array:

$ignore

You could modify that to do something like this in your script:

$ignoreuserid = $vbulletin->userinfo['userid'];
if ($ignore[$ignoreuserid])
{
	print_no_permission();
}

Though you'll probably need to use the following for the array:

field ignorelist from usertextfield database table
instead of using $vbulletin->userinfo['ignorelist']

Depending on your code, something like this should do:

// get ignored users
$userinfo_ignore = $db->query_first("SELECT ignorelist
	FROM " . TABLE_PREFIX . "usertextfield
	WHERE userid = " . intval($the_userid_that_started_the_private_conversation)
);

$ignore = array();
if (trim($userinfo_ignore['ignorelist']))
{
	$ignorelist = preg_split('/( )+/', trim($userinfo_ignore['ignorelist']), -1, PREG_SPLIT_NO_EMPTY);
	foreach ($ignorelist AS $ignoreuserid)
	{
		$ignore[$ignoreuserid] = 1;
	}
}

$ignoreuserid = $vbulletin->userinfo['userid'];
if ($ignore[$ignoreuserid])
{
	print_no_permission();
}