Assumptions baked into the VMS carbon dating

[Stefan Wirtz_2]

[..] Vellum is a non-perishable commodity, like timber, leather, copper, or vitriol, so a merchant's stock could well have new vellum mixed with decades-old vellum.  Especially considering that the Author was happy to take off-cuts and grossly defective sheets. 

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, even more when this vellum is endangered by parasites, humidity and decay. 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.
As long as there is not one example of such „business practice“, I can‘t assume the story of „VMS was written very much later, on then older vellum“.