You can either use the manufacturer-supplied driver or NI-VISA to control your USB device but not both. If you have installed your device, each time you plug it in the FTD2XX.inf will associate your device with FTD2XX.sys. Then, you can use FTD2XX.dll to talk to your device using the call library function node. The DLL usually contains a whole bunch of useful functions that someone else already designed to talk to your device. This is the easier route and I would recommend it if you already have the DLL and the installed driver.
The second method would be to uninstall or disable the driver for your device and create an .inf file with NI-VISA Driver development wiazard. This .inf file will tell Windows to use VISA as your device driver. You will, however need the vendor and product ID of the device to use VISA as your driver. The details of this are outlined in the NI document "Using VISA 3.0 to control your USB device."
The drawback of this method is that VISA is just an API to talk to your device and you have to implement the communication protocol to talk to your device. This might take some time to do.
This is why I recommend the first method. Someone else already implemented the driver for your and you just have to use the functions from the dll to implement your application.
Hope this helps.
