Ignoring challenges such as primitive data-type mappings, complex data-type mappings, and non-existent data types across the two platforms (Windows/.NET and J2EE), the easiest way to communicate from VB 6.0 to J2EE is via SOAP documents.
In Visual Basic 6.0, you can create a SOAP document using either Microsoft's SoapClient, found in version 2.0 of the SOAP Toolkit, or by manually creating the SOAP document. Using the SoapClient, you can not only create the SOAP document, but you can also communicate and send the SOAP document to a J2EE Web service.
Manually calling the Web service from VB simply entails connecting with the endpoint (the Web service's URL) and sending the SOAP document.
Microsoft's Web site provides a very comprehensive guide to this very subject entitled, "Application Interoperability: Microsoft .NET and J2EE".