Just curious, the heating to remove the VRM is that permanent so you can only check the authenticity once?

Yes, you are right, once the VRM is erased it is gone for good.

Though it is possible to test only a fragment of an object, but I do not know if the instruments that were used were sensitive enough to work with only a very small chip, which could be glued back, afterwards.

Many such ancient pottery objects are already broken, so they must be glued anyway, if a restoration is desired.

It strikes me as risky, if the item is in any way delicate or has absorbed water at all. But I'm no ceramics expert!