I'm not quite sure what you are working on, but I can try to help you out. Let me see if I've got the idea. You have a C++ DLL that you want to call from LabVIEW and the function you are calling requires a variant as input? Is this correct? Does this cause LabVIEW to crash? Let me know and we can try to work at this.
Also, if it would be easier for you, you can contact your local NI Branch office if there is one. They can provide support in the native language, so that may be easier for you. For instance, if I were in Germany, I would contact germany@ni.com. Let me know if I can help you out.
J.R. Allen