I have managed to use the get_node_values call back for vmware 5.03 with great success. You can easily pull out nodes in the tree using the notation: -
<vCenter IP>:<host UID>
<vCenter IP>:<Cluster name>
The same call back for the 4.20 version of the probe is not as co-operative. And it uses mkey and and nkey parameters which I can't get to work. Running the call back with just the resource filled in returns the next node down which is Datacenters. I cannot get any further. I can't find the notation for the Datacenters node so I can't continue down the tree.