I think maybe the VFD USB version will never come.
VFD use much more power than LCD, a 20x2 VFD will draw about 200-300 mA. 20x4 VFD will consume more than 400 mA i guess. The usb standard only allow 500mA per device in bus power mode. So, a 20x4 vfd with some GPO should over the limitation. And the device may kill your USB interface.
VFDs are mostly used in industrial style situations, where high visibility and high tolerance of abnormal operating conditions are required. This market doesn't really need USB displays.
As jar says the main issue is power, I thinkt he 20x4 might even use more than the 500mA USB max
So MO will probably have to make drawing power from a separate power connector mandatory for the vk204 USB controller... though I'm sure they'll make one (eventually).