Plus, a vest doesn't cover the head and limbs. The rating is an abstraction. Stress is an abstraction. I think it's not very useful to try to equate the rating of armor with stopping power in the real world.
This is true. I still think that heavy, bulky, military grade body armor should be a level 3 - same with a full coat of plate vs melee attacks.
The tradeoff is that there is no possible way to say... go to the mall wearing either one of them without drawing attention.