The proof of Rav Shach that the Rambam hold by the Rashbam is hard to understand.
His main point is the fact that the owner of the object can ask for the pieces back.
The point is that if the Rambam would be holding like Rashi and the Rosh that the thief must pay back unbroken vessels or money, then paying back the broken pieces does not fit with that. But the way I see it neither does it fit with the Rashbam. If he can pay back any שווה כסף (anything that is worth money). Then what gives the owner the right to ask for those piece specifically?
That is one way or the other we need to find some reason the owner can ask for the pieces back. But what ever that reason is can not have anything to do with the argument between the Rashbam and Rashi and the Rosh.
This was the last idea I had before I broke my leg. [I called for help and some people called an ambulance and took me to the local hospital.] ]