So all items are handled narratively in the system, and don’t usually have a mechanical effect other than letting you do whatever they would do. Having a gun let’s you shoot, when you otherwise wouldn’t be able to. With the body armor, that has worked so that in a situation where you would take a Hit from physical damage on a failure, there is some grace in the narrative worked out by the GM and players. Say if the player said, “hey I put that body armor on, I want to charge down the hall where the guards are firing and to the next hall.” The GM could go, “Alright, if you fail you won’t take a Hit this time, but you still won’t get to where you are trying to go.”
I think a simple mechanic of it absorbing the next Hit would make sense, there just isn’t a mechanical system for gear at this point. It’s something I’ll start looking at. Also, in your game when running it, there is the flexibility to make those calls as you see fit.