A quick search on this site shows a number of threads where people have swapped their body onto a different frame (same vintage, but better shape, or newer like an S-10).
I believe on most of our trucks, the VIN is stamped on the frame by the steering box. Let's say, you cut the front of the frame off and put a front clip off a Camaro or something else on it, Now there is no VIN on the frame.
That said, there are a number of scenarios where the VIN may not be completely accurate on these old rigs, especially the older ones that may have been modified and hot-rodded!
In my opinion, it is always best to check with the local DMV to ensure you are not creating a can of worms. Vintage travel trailers are even worse since a lot of them never had an actual VIN and a lot of them are bought and sold with no title!