This is the acctual URL of the API
Example:

/severe_messages /count

/severe_messages /get

/sessions /count

/sessions /launch

/sessions /get

/runs /compact

/runs /update

Please see the full API list at http://[vAPI HOST]:[vAPI PORT]/vmgr/rest/$docs

Please note that the output of the API call will get saved as jSON format into the workspace at ${BUILD_NUMBER}.${BUILD_ID}.vapi.output