Hello Santy,
I've tried zabbix as monitoring tool, good one with nice review but quite cumbersome to configure. Well, i guess once SNMP is enable at ESXi level, the discovery part is done at Monitoring tool level which needs to be properly configured. Try to check zabbix settings for snmp.
https://www.zabbix.com/documentation/2.0/manual/config/items/itemtypes/snmp