Yes, I thought of that quite a bit on and off over time.
One important difference is that the optical sensor circuitry needs (a little) power to operate, which a potmeter-based interface often doesn't provide (not enough mA), because the potmeter is just a voltage divider with next to no current flow.
I first tried a Kawai F-10H on my Roland's pedal input, and got some response but not a good one, for that same reason.
I then bought a single-input MidiExpression interface (which is Teensy-based (or similar microcontroller)). That one also couldn't provide enough power for the Kawai pedal. So I corresponded with the maker of the MidiExpression about that and we both, independently, devised a solution. Mine and his both worked (I tested his prototype) and both produced the exact same MIDI output. (Both solutions weren't exactly rocket science 😄, but because that's not what I usually do I'm still proud I cracked that nut).
Since then, his interface(s) happily support(s) the optical Kawai pedals too.
To conclude, interfacing a GFP-3 unit directly to one's computer using an interface or one's own little microcontroller project is probably more feasible/logical than to try to make the VPC feed the pedals and adapt to the output of the optical circuitry.
Cheers and happy interfacing,