Writing userspace USB drivers for abandoned devices

I recently found some USB devices on eBay (Epiphan VGA2USB LR) that could take VGA as input and present the output as a webcam. Given that I was keen on the idea of not needing to lug out a VGA monitor ever again and there was claimed Linux support I took the risk and bought the whole job lot for about 20 (25 USD). When they arrived, I plugged one in under the expectation that it would come up as USB UVC Devices but they did not. Was I missing something? Turns out that he was, and that was the start of a rather wild ride.