We have one of our test systems working successfully using the Prelive environment at https://payments.vantivprelive.com/vap/communicator/online with appropriate credentials provided to us by Worldpay. However, we have not been able to get Postman to work successfully against that Prelive URL at all. It just spins for a while and then we get an error response indicating it's either a server couldn't send response, or SSL Connections being blocked, or cookies not being sent, or request timeout.
I have followed steps in the Young Developers - Using Postman to test APIs article. That works but it is using a different URL -
Note: Postman works fine against the Sandbox env at http://www.testvantivcnp.com:80/sandbox/communicator/online.