There are a lot of different ways you could go about this. Probably the most painless way would be to use zabbix 1.8.x's ssh login ability to run a simple shell script on the vmware server that would return the result to you. You have to compile zabbix_server with libssh for it to have the capability to do this.
Once you've compiled zabbix server with the ssh capability, just go to the vmware server "Host" and add an "Item". From the drop down list will be "SSH Agent". Choose that, have it login to the vmware host, run the simple shell script which returns the space utilization of the datastore.
Another possibility would be to install zabbix_sender (or a homemade equivalent) on the vmware host and have it periodically send the datastore results to zabbix_server.