They did use the same physical part. All C128s report 16k as far as I know, even the 64k ones. Anyway, they did different ROMS over several machines. It's not that hard to program them with a slightly different image. The 64 needed a different one to the 128, so having 2 models of 128 wasn't that hard.
Oh, I know that it would be a very small savings in the end, but recall they were using mask-programmed parts rather than electronically writable ones. Even with the chip foundry nominally part of the same company, that would have to be a minor logistical nuiance. Of course, from what you say about them using only the one ROM regardless of how much VRAM was present shows they actually took the even more tightfisted route of simply not bothering at all. I wish I could say that surprised me, I really do…
The world’s only gsteemso
Agitator-in-chief for the Seattle Retro-Computing Society
All I can say on the "mask rom" thing is that my early 128 actually had an EPROM in it from the factory.
But yeah, Commodore certainly shaved those pennies off where they could! I think almost every 128 shipped has a bodge wire fix because that worked and there was no point spending the time/money to redesign the PCB to remove it