A 155mm hit I am skeptical. But I did see the Future Weapons he is talking about (even though I hate the narrator) and I was impressed by vest.
They showed a test where the vest survived a grenade blast at point blank. I doubt the wearer would have survived the blast however.
I can't remember where I saw it, but I do remember an article, pretty sure it was Vietnam era, of a guy that dove on a grenade with a vest on and survived it. Had pictures of the plate and the nasty bruising on his chest. He had a few broken ribs and likely a few other injuries, but he was alive...