Unfortunately the VIN number doesnt do any good in making duplicate keys, otherwise anyone could write down the vin number for your vehicle found at the left front corner of the windshield and have keys made and steal the vehicle.
The only way to have a key made when the existing key is no longer available is by the key code, which back in 1989 was on a small rectangular stamped pellet found with the owner's manual and on file with the car manufacturer.
GM doesnt keep those for this long and as you found out.
Unfortunately these leaves 2 possible choices.
The first is to have a locksmith come out and remove the key cylinders and check the tumbler pattern and make new keys that way.
The seconds choice is to replace the key cylinders with new ones that come with their own set of keys
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