The VIN number on a Defender is found on an aluminium plate rivetted to the brake master cylinder/pedal assembly under the bonnet, and also stamped into the chassis on the right hand side, outside of the chassis rail just behind the bumper. The ally plate will transfer to the new chassis with the rest of the bulkhead. Galvanizing will actually pick up on the stamped numbers on the chassis causing the digits to be raised above the surrounding surface. Very odd. Something to do with surface tension in the hot zinc? Fitting a NEW chassis causes no issues with identity, but if you find a good used one, the identity of the vehicle becomes suspect in the eyes of the authorities. Grinding off the old number and stamping a new one over it is detectable. Cutting off the bracket to which it is stamped, and welding on the one from your old chassis is not (if the welding is good), but be aware that Land Rover chassis' have date stamps elsewhere which can be found by those that know!
As far as border checks go, the vin plate on the brake pedal box is usually enough, but sometimes a stamped chassis is all they will accept.
Hope this helps someone, probly a bit late for Roamingyak though.