That's unfortunate, I always heard Vredestein tires were a good brand, with a good warranty and good reputation. They're manufactured in the Netherlands, and in general, the Dutch produce good stuff. Of course, it's always possible you got one of the few which slipped through Quality Assurance unoticed.
But, playing Devil's Advocate for a minute (which you're sure not to appreciate - sorry in advance), a Tire is a consumable item on a car - they all wear out eventually.
The Tire manufacturer must impose a specific set of requirements in their warranties to keep from going broke honoring bogus claims. This is as true of Vrdestein as it would be for Michelin, Pirelli, Goodyear or Bridgestone.
The Vrdestein Warranty lists several requirements of the owner -
http://www.tiresunlimited.com/ALL%20TIRES/Vredestein/vred_warranty.htm and these aren't really any different than those of other mfgrs.
Among these, they require that a sales receipt be presented along with the claim - that's reasonable as they don't owe a thing to anyone but the person who actually bought the tire.
If you don't have a receipt, that's not their fault - look up your old CC bills and contact the retailer to get a copy or other documentation denoting you as the purchaser. It may be a hassle, but it falls under your responsibility, not theirs.
They also require that the Tires be rotated every 8,000 mi. and that proof of this be presented with any claim. Again, a reasonable requirement to insure that you were properly maintaining the Tire during your ownership.
If you cannot present this, again, you cannot fault them.
Also, they require that you keep the Tires at the proper inflation. Also reasonable because if the tires are not properly inflated, they can fail in any number of ways.
Vrdestein makes their Tires for a variety of different car applications, so they must design a pressure range into the Tire. The Tire will state a Maximum safe Tire pressure (DOT requirement), but that doesn't mean you should run at those pressures, it depends on the car and most specifically it's weight.
You state you keep the Tires
never below 38 lbs. Tires pressures for the Boxster are spec'd at 29PSI Fr. and 36PSI Rr. So, you have been running the Tires over inflated (esp. the fronts) - a violation of the Warranty requirements.
Over inflation, aside from causing early center tread wear, can result in excessive heat being generated in the Tire, and one of the problems this can cause is a separation of the tread cap, which is what appears to be happening to your Tires.
The reason you want to go with the car mfgr. recommendation is because this pressure is determined to meet the performance the car was designed to and also insure that the Contact Patch is even and maximized. Sure, people play with pressures to change the handling characteristics and such, but you do so on your own accord. You cannot expect the Tire Co. to shoulder responsibility for your actions, especially after the fact.
The Vredestein warranty also has a time limit - 5 yrs. from date of purchase. Again, reasonable as Tires change over their life due to use, exposure to UV, Ozone, and chemical changes in the materials. Vredestein cannot warranty their Tires past this point, because they have a high liklehood of failure due to age.
You don't state the age of the Tires, but if more than 5 yrs., you cannot fault them for failing to honor a warranty they stated up-front had a 5 yr. expiration.
I'm sorry, but the case you describe is just not Black & White. Like it or not, you may have caused the early wear. I think slamming the Tire co., and all it's products as you have may be a little over the top.
Perhaps you should get your ducks in order (purchase receipt, proof of rotation, etc.) and contact someone at Vrdestein directly -
http://www.vredesteintire.com/ and work your way up the chain. A change in approach is likely to benefit you as well. Good luck!!