Assumptions baked into the VMS carbon dating

[Jorge_Stolfi]

There is no reason to believe that a medieval merchant stocked those „long-lasting“ vellum batches for decades, even exceeding his own lifespan. This would have been dead capital.

Compare that to the lumber or leather trade.  A merchant would normally keep a stock of hundreds of whole skins and/or pre-cut sheets of each quality and size.  When he sold or bought another batch of some type, he would not have to follow a strict FIFO policy; he might as well randomly mix the new and old sheets.  So the sheets in each stock pile could end up having vastly different ages. The old ones would not be any more "dead capital" than the new ones; they would be just "capital".

Argument of defective/low-quality material is even worse: no tradesman would keep such product for 20 or even more years in his books and shelves

Like for timber or leather, there would be a market for off-cuts and "reject-quality" vellum: for small-format booklets, for stuff that had to be permanent and waterproof but did not have to look good, for scribal practice, for covers and binding strips, ...

Merchants and manufacturers may or may not have stored off-cuts and "reject-quality" material.  Merchants may have acquired whole skins, cut them into standard-size sheets, and kept the off-cuts and bad pieces for when and if someone (like an old impoverished trader returning from China) wanted to buy them.  Throwing those pieces away would still be throwing money away.  Vellum sheets, good or bad, may have gone through several wholesale and retail merchants before reaching the final consumers.

even more when this vellum is endangered by parasites, humidity and decay.

But the big attraction of vellum was that, like leather, it was resistant to those threats.  The tanning process made it generally indigestible by insects and unaffected by humidity.  

The worm holes that we see in the VMS seem to be all on places where water spills could have softened the glue from the covers and transferred some to the adjacent vellum sheets.  Unlike the vellum itself, typical glues (hide glue, gum arabic, library paste) are edible.

All the best, --stolfi