I agree with you, in looking to purchase a vintage timepiece, any brand, one must conduct through research before buying. I would call that a prime example of Caveat Emptor, or "buyer beware". There are many sources of information, and confirming the serial numbers is one. But I can tell you that there are errors in the records. Also, and this is really odd, they, for some reason can't look at just a case number and tell you when it was produced or issued nor the corresponding movement number. Now I can see that as a policy to prevent piracy, they would not want to give out the matching serial number information. But when it comes to a watch that is obviously correct, or otherwise outstanding, this should not be the case.
Just in the case of Dean's example here, looking at the inside of the back case I can tell that something may be wrong. JLC used the same cases, but marked the serial number on the outside of the case back. Also, the movements can be faked with putting a Vacheron bridge on a un-refinished chrono movement. But other than that, the hands and crown are correct, and the dial is refinished, but otherwise correct. In this instance I would say it needs more evidence, but even if correct, it would be on the bottom end of the pricing due to the dial and case being polished.