Do you need to set some kind of Key, Value info up on a VirtualMachine in vCenter that you can access from the API? I thought with 5.5 and the addition of this that there was going to be some kind of actual Matadata fields added to the vSphere API, but it seems that still hasnt happened, as this is incomplete and missing any way to actually use it. The good news is that since 4.0 there has been a way to add key value pairs to the vm.config.extraConfig. The key has to be a String, and the Value can be an object. I have used this to store all kinds of stuff from Strings to JSON and XML payloads and even base64 encoded files. I wrote a sample for the pyVmomi-community-samples project that uses the vm.config.extraConfig to store some key value pairs. You can get it here until it gets accepted by the project and merged into master.