I think there are a couple of chalenges to any vpn-like organization/system. First, the certification is based on your pizzeria meeting the standards at the time of certification, and perhaps at regular intervals. What's to stop a certified pizzeria from 'loosening' the standards while making pizze for the customer? Secondly, it costs money to become and stay certified; it is to the certifying organization's benefit to 'certify' as many pizzerie as possible. Loosening the standards or their enforcement means more money (within reason, of course; the end product needs to be in the right ballpark for their name and therefore their economic viability to stay intact).
A vpn-specific problem, in my opinion, keeping in mind that I'm no expert, is that they pay too much attention to the process as opposed to the end result. If my pizzeria uses a spiral mixer instead of a fork mixer, but I make pizze that are identical to Trianon's, then why can't I be vpn? Likewise, why can't I be vpn-certified if I am using a cold-fermentation technique if my pizze are just as good or better than a vpn-certified joint?
I understand that the avpn is trying to protect the artisinal process, which I applaud and encourage; but don't do that at the expense of the finished product. Why not protect the process and the product? Of course, that would involve certifiers visiting the location and actually trying the pizze an a regular basis, which is not realistic.
In the end being vpn-certified is good marketing, and not much else.