much better to do the VIN check BEFORE seeing the car - you will then know more about what to expect when you actually visit and you know in advance what to test on the car.
Just ask over the phone for the VIN - it is there for all to see anyway.
..and make sure you check the VIN on the reg document, in the windscreen viewing panel and also on that chassis. They should all match of course and not look as thoughh they have been tampered with.
In not a cop but theres a system we use at dealers that will give us chassis from reg i could cross referance a chassis number BUT we will not jus hand over chassis numbers for particular reg numbers. Its not often we are asked to ths only once ive been cos the buyer of the van wasnt convinced all was it seems.