Hello, Great project! I'm not a total expert in Whatsapp. But I have worked with many APIs for systems such as OAuth, SMS texting, airtime, Fitbit, Facebook, Twitter using VBScript/ASP.
We could write this as a Module or Class depending on your needs.
I suggest we develop this systematically.
1) We create a "dummy" CLASS with all the methods and events you require. It will contain test data, and respond to test events.
2) On acceptance of this CLASS, real implementation code will be written. NOTE: a URL endpoint on the internet will be need to recieve messages.
The CLASS can be eaily inserted your VB6 project.