The feature vacuum of the zabbix in the new versions PostgreSQL is generaty overhead in the server, because the zabbix execute manually vacuum in the housekeeper table. When it monitors very itens (200 or more) is occuring two or more instance of vacuum the same table (housekeeper). I believe that execute vacuum is responsibility of DBA and execute outside software (zabbix) with jobs or task.

Also The PostgreSQL 8.3 and major has by default feature autovacuum that execute vacuum in the better time and don't have overhead in the server. I remove feature vacuum of the Zabbix and the performance improved considerably to build zabbix without vacuum.