The only vintage Vader case fake/repro prototype cases are the Gold Chrome on black plastic cases. A few years ago in Toy Shop, there was a dealer making repro's. They are very easy to tell apart from the real thing, but shady dealers picked up on them and resold a few of them as legit.
If there is ever any doubt, contact me or another vintage collector.
Bill - the fakes were they just gold "plated" on the exterior, but the interior was left black? A vintage toy shop I visit has one such Vader case buried under a pile of junk behind a display case (it's been there for at least two years). I've always figured it to be a fake, but I'd find it kind of funny if it was one of the Kenner test pieces and the owner doesn't even know what he has. I've never seen interior shots of the test pieces and can only find exterior pics on the Archive with no references to the interiors.
All of the known fakes/repros are the black cases w/ the gold chrome. This is NOT saying that all such cases are fake. The original chroming tests by Kenner were done using the black cases. (this is what they had on-hand)
As the process evolved, Kenner started using different plastics (ie. the Dull Gold Case), and some of them being Chromed as well. In all, there are 3 main variants for the Gold Chrome case.
Your best bet it to examine it, and see if it has been split/cut along the main seam for the case. (Kenner cut the cases to aid the chroming process.) Also, look for general signs of wear and aging. If it is a black case (whole) and has a very deep gold color, without a lot of wear, then it is probably one of the vintage repros/fakes.
If you can post some pics of the case, or send them to me I can give you a better idea.