GOT IT!! Okay, two things I had error with:
1) In that last couple attempts, I had the body wrong - I had specified input-parameters instead of "parameters"
2) json value format
After updating those two and re-adding the Content-Type=application/json:v5.1.1, my simple test using the RESTClient in Mozilla Firefox successfully launched the workflow in vCO 5.1.1 Virtual Appliance:
