Putting a vin number or any other type of ID on a converter is a waste of time as previously stated. If they have the ability to remove the converter they have the ability to remove any such markings.
Until we as a society are willing to convict and put thieves in jail for lengthy periods of time for stealing catalytic converters, jewelry, high end clothing, and appliances it will not stop in the near future.
There should be stiff penalties for anyone who accepts a stolen converter. But one problem we have is our open border when they can transport the converters to Mexico.