This is where web services shine. Integration, integration.
If you expose business logic of the "web application" as one or more web services, then you can simply call the services by sending
SOAP messages from the VB application.
Web applications that are built using Struts typically are GUI based applications that are used by a human that is using a web browser.
If you want communication from application-to-application, you may need to reengineer the business logic of the web application and expose it as one or more web services.
If you coded all the business logic in Struts Action classes, you did not use Struts correctly and will have more work to do implementing web services.