How do I parse application/vnd.surveymonkey.response.v1+json?
So the answer is to use hook_rest_server_request_parsers_alter(). This way, the application/vnd.surveymonkey.response.v1 Content-Type can be mapped to the standard JSON parser.

function MY_MODULE_rest_server_request_parsers_alter(&amp;$parsers) {
  $parsers['application/vnd.surveymonkey.response.v1+json'] =  'ServicesParserJSON';
}