First of all, the items that can be fetched with zabbix from vsphere servers are very limited.
If you want to fetch additional items from ESX-Servers, you can use the vSphere CLI – a perl toolkit with several handy tools.
One of the tools i can recommend from the toolkit is performance.pl
You can fetch almost all available items from virtual machines and feed it to zabbix. We are fetching the cpu readystates to monitor the machines and decided whether they are overcommitted.