To go any further, I will need to see photos of the engine VIN & the body numbers.
It sounds like the seller may be trying to pull the wool over your eyes.
Dash VIN tags were made of high quality stainless steel. If the VIN tag was in fact "destroyed in a fire" the body, since it is made of regular steel, would have collapsed in on itself before the dash VIN melted.
Buying a car with a missing VIN tag is something I would avoid personally. If it came with a clean title and a state issued replacement VIN, that would be better than nothing though.
In this case, if the seller had a title, the VIN would be known. If it were me, I would turn away from this deal and fast.