Sample Exchange

vSphere_Machine_1 is just the ID that vRA gives the vSphere machine within the composite blueprint. If you go to Design / Blueprints then open your composite blueprint for editing, then select your vSphere machine in the Design Canvas, you should see "vSphere_Machine_1" or "vSphere__vCenter__Machine_1" in the ID field.
To get the ID dynamically rather than hardcoded, you would have to parse the request template ($Form.data) to find the ID.

Where do you get vSphere_Machine_1 from? I want to be able to go from a catalog name item to the full request without having to hard code something like this in there.