Windows Service written in .NET accessible from VB6
See the question and my original answer on StackOverflowThe WCF service is still an option, but you would have to ensure the WCF configured binding is interoperable (choose BasicHttpBinding for example as seen here: WCF Bindings In Depth).
On the VB6 side, you can call an existing SOAP endpoint, as described here for example: How to Consume a SOAP Web Service in VB6?