There is no built-in API and SDK in VB6, so you need to wrap it by your own. There is a SOAP Client for VB6, called as Soap Toolkit. But it is for simple request structure. The AWS requires a more complex data structure in its request so you should use SerializeEnvelop method where the request formatted in raw XML and then post to the AWS URL as well. This will warrant you will perform the correct request just like what .NET does. And the response would be in XML structure which can be easy in parsing by using XML DOMDocument object.
Let me know if this method is acceptable.