Hi Arnaud,
If you need to call VB functions from a LabWindows/CVI application to control the tested devices then you need to create a VB DLL that you will call from LabWindows/CVI. As VB DLLs expose their functions in a particular way, you can't call them directly from LabWindows/CVI and you will need to create an ActiveX controller.
To have more infomations about it :
http://digital.ni.com/public.nsf/websearch/AA35D259FD350D3E8625699F0059C641?OpenDocument
Best regards,
Benjamin
Application Engineer
National Instruments France