Thanks for the quick response! The SDKs you referred to appear to be focused on client development. I'm not developing a client, but rather an API extension using Extension Services. In short, I don't want to "communicate with vCD", I want to extend the vCD API so clients can communicate with my service.
I've also been digging in the development center, but there isn't much there on API extension development, and what little was there is over a year old.