How are you storing it?
IIRC, the session array is for things from the session table, so if you are setting it via cookie you should use something like this,
PHP Code:
$dialogDisplayTime = $vbulletin->input->clean_gpc('c', 'dialog_display_time', TYPE_UINT);
Then access it via $dialogDisplayTime or $vbulletin->GPC['dialog_display_time']
(to set the cookie, use...)
PHP Code:
function vbsetcookie($name, $value = '', $permanent = true, $allowsecure = true, $httponly = false)