I bet rules will also be different according to where you are. I registered a Sankey trailer here in CT not too long ago and the DMV wouldn't accept the VIN number I gave them. All I needed to was get a VIN check. This involved me bringing it for inspection by an officer. Both he and I combed over the trailer and decided to use one of the numbers found on a plate. I don't think it's really the VIN number but being that it was so old, the officer didn't really care all that much. Now it's official or at least until it gets sold out of state.

Not sure what would have happened if it was a car but the impression I got was that the CT DMV didn't really care if the vehicle was older than 20 years. YMMV.