Well then, I'd say you need a different approach, or to be talking to a different person. If you could get a letter or such from a qualified mechanic, that couldn't hurt either. Try to get hold of their Customer Relations people, they are sometimes more amenable.
I once was in a similar situation as you (not tires, but another warranty dispute) and was able to get some interest (when everyone else stonewalled me) by contacting the V.P. R&D Dept. directly. The R&D guys have a vested interest in how their products perform. In my case, not only did they send me a direct refund for the defective product - $950 (totally skirting the Warranty altogether), but they sent me the newest updated version of the product FREE to 'Beta Test' 5 mos. before it was released for sale. Don't know if this would work for you, but it couldn't hurt.
I know that Michelin and Dunlop (possibly others) have an arbitration procedure set forth in their warranty, this may be an option, though possibly more hassle and possible expense.
You could also try writing to the The Royal Netherlands Embassy - Washington DC -
http://www.netherlands-embassy.org/NABID.asp and present them with your documentation, that might help as they're always bringing in Trade Ambassadors to promote their country's products and services - they don't like negatives or dissatisfied customers. They may intercede in the dispute.
I'm just still confused why Vrdestein would put up such a fuss. This is really the 1st truly negative thing I've ever heard about them. Good Luck!