In 5.1.1 :
If following header is provided
"Accept: application/json" - Returns JSON response as it was in 5.1 GA. This format is deprecated and will not be supported in newer version of VCO.
"Accept: application/json;v=5.1.1" - Returns JSON response in new format allowing posting data back to the server. This is the format that will be supported for the next version of vCO.
Botom line is that in 5.1.1 you need to have "v=5.1.1" in accept header in order to use the new format.