You're going to get a lot of mixed opinions here when it comes to VPN, and, to be honest, I, personally, like the idea of Neapolitans forming organizations that protect their culture as it traverses the globe, but I think many believe that VPN is mostly about making money rather than protecting culture and spreading knowledge.
Feelings about VPN aside, I think the most important aspect of choosing training is the teacher. In other words, ignore the organization that's doing the 'certification' and take a long look at who's actually going to be teaching the class.http://anticapizzeria.net/vpn/about-us.html
I recently met Giulio and had a chance to sample his pizza, and I can say that unequivocally, he's the real deal. I didn't get a chance to talk to him much- I would loved to have gotten a better idea as to how open he is about his process. A good teacher is an open book. From talks we've had with Roberto, it doesn't seem like he keeps any secrets- in that sense, he's more teacher than businessman. If Giulio is equally as open (and I'm guessing that he probably is), then 1,550 for three days of training, for someone that wants to streamline their Neapolitan pizza making education and for whom $1,550 isn't a lot of money, then I think he'd be a good choice. This is all moot, though, as Giulio is very actively running a relatively new pizzeria and is most likely not traveling to LA to teach. I know very little about Jose' Barrios, so I can't vouch for him.
There are definitely less costly ways to learn Neapolitan pizza. Matthew and Craig are both self taught and capable of making renowned pizza (along with other members of the forum). Even with 3 days (which is very little time), you're still going to need countless hours in front of an oven, so a huge portion of your learning is going to be self initiated anyway. If you develop some skills on your own, there's also apprentice opportunities to be found. There's also reams of information within these walls.
If I was venturing into the world of Neapolitan pizza, I wouldn't spend a cent on training, but I'm pretty cheap and have a very gung ho DIY disposition. For those that are looking for a little more hand holding and don't mind spending top dollar to get it, then training might be a viable option. But, as I said, make sure it's with someone that knows their stuff- Roberto's super busy and isn't doing training right now, but I trust him as a teacher, and, while Giulio might not be traveling to LA, it might be worth investigating his availability for classes in NY. Matthew might be venturing towards training, although I've heard he might have issues with oven access during off hours.
The most important aspect, like I said, is the teacher. If you really want to go the training route, find the best teacher, regardless of organization affiliation, and make absolutely certain that they're going to be present for the entire class and not just showing up now and then to make sure their 'assistants' are doing a good job.