Dean,
Are the owners looking in the correct spot? Have 'em open the hood and stand behind the passenger side, front wheel. Then look down at the space below the windshield wipers, that is covered up when the hood is down. The tag is right there, or that's where it should be. There is a rectangular hole stamped into the cowl panel so that the trim tag can be viewed. If there is no tag in that rectangualr "hole" than it's gone for some reason.
The VIN, like Tom said, will only provide basic info. It only tells you the year, the body style, what the final assembly plant was and the sequential number for the car. That's it.
If the trim tag is gone it will take a little more work to prove it is an SS. Proper bucket seat brackets, console, swirled trim inserts. If it is a powerglide car it will have the floor shifter and, as such, will have the wires for the neutral safety switch passing through the firewall, just southwest of the master cylinder. It's a hole about 1 1/8" in diameter and will have a rubber grommet with two wires passing through it. A 4-speed car will have a 1" hole with either a grommet or possibly a plug (where the dash harness and back up harness to the trans plugs together) with a single, "double" wire. It looks like a lamp cord. The back up harness is dependent on what transmission it had. Either the T-10 or the Muncie. You could get a 3-speed with ALL engines in a SS in 1963. Yes, even the 409s. The 3-speed was still shifted from the column in the SS.
All engines 300hp and up (300hp 327, and all 3 409s), with a 4-speed, WILL have a factory tach. It was a mandatory option. Optional on the 250hp 4-speed cars. But that's true for the Impala's, too.
Post your findings or pictures. I'm sure we can help you out!!
Ross