I've seen a few cars put together from parts of others. That mixes up the VIN numbers pretty well.
Any signs of non factory welds, or seam sealer anywhere under the hood? Could be they did a simple front clip, or used some parts off another car, and if the job was done right, your fine. (Does not weaken the car. If anything, it's stronger if the job is done right.)
Most insurance companies will not replace the cowl or "firewall" of a vehicle. (Where the main VIN plate is attached and in many cases, stamped into the metal too.) There are hidden VIN's all over the car. They don't even let me know where many of them are, but experianced body men know where most are since they see them while replacing parts of the car. Hidden VIN's are often on the inside of panels you can't see w/o knowing exactly where to look, and it takes a mirror in some cases.)
I'd get more information, and make sure you looking at VIN numbers, and not some other information. If the numbers really don't match, your car/chassis is likely a prior total loss. (Or really more than one of them.)