reverse "VGA ADC" in what exactly?

well you can buy video ADCs from parts suppliers (digikey etc, some parts are pretty cheap, but again, it was seldom clear if they could be used continuously or would refuse to work without proper VSYNC etc) or source them from older digital monitors/dvd players/VGA2HDMI etc (roughly anything that has a VGA in)

fast ADC is not the problem per se, you can buy $10 ARM parts with 80MHz 12 bit ADC (LPC4370), pumping that data into computer fast enough is.

such a scope would not be a raw part, but include an FPGA: summarize the triggered traces that happened since the last transfer to PC... alternatively speaking, consider the amount of information on the screen of an oscilloscope, thats a low color count, with relatively large areas of the screen unaffected, so 60fps of very compressible image data..