'The Home Computer Wars' has this to say about the development of the cartridge modem for the Vic-20:
The size of the case was another problem. Most modems came in bulky rectangular boxes about 10 X 4 inches. Too expensive. I suggested we put the modem on a cartridge and plug it straight into the VIC-20 user port. Again, they went back to the drawing board and somehow squeezed the circuitry into a case less than 3 X 6 inches. It was the first modem-on-a-cartridge. The best part was, our competitors couldn't duplicate it. Only the VIC-20 had the right combination of RS-232 interface and user port engineering.
I thought the RS-232 port on the Vic-20 was completely separate from the cartridge port, so a thing had to be plugged into one or the other, and if the modem was plugged into the latter then it could not use the former. What am I missing?
I have to admit I don't recall hearing about cartridge modems for any other machine. Is this why?