I get redirected to your site with this url -
http://afterworld.ru/rss.xml
I don't think you need to globalize $vbulletin there, I know I don't on the couple of cron jobs I've written. Do you have a table prefix? You should probably write the query as?
PHP Code:
$vbulletin->db->query_write("UPDATE ".$TABLE_PREFIX."post SET showsignature = 1");
And I think it will only log if you have Log Entries set to Yes.