Symptoms

• The client was successfully authorized on the Swagger page:
  Vantage processing REST API Swagger (vantage-us.abbyy.com)
  Vantage processing REST API Swagger (vantage-eu.abbyy.com)
  Vantage processing REST API Swagger (vantage-au.abbyy.com)
• All scopes were selected when authorizing.
  
• A "custom" client_id and client_secret are used (created in the Configuration > Public API Client section on Vantage Portal).
• All API requests return 403 responses.

Cause

There's a known bug with how the scopes are granted on the Vantage processing REST API Swagger page. SwaggerClient gets the "openid permissions legacy.client" scope, which allows it to send requests in Swagger. 

The "custom" client created in the Configuration > Public API Client section on Vantage Portal gets the "openid permissions" scope resulting in 403 when sending the requests.

Resolution

The default SwaggerClient can be used to interact with the API via Swagger.