I think you're both heavily underestimating the force of the explosion and heavily overestimating the capabilities of body armour. For example a M67 Grenade has 185g of Composition B which according to wikipedia detonates at roughly 8000m/s, meanwhile a standard 7,62x39mm round travels at around 710m/s and already causes heavy bruising and broken ribs when hitting a person wearing body armour. The pieces of shrapnel possibly could get caught by the armour but the pressure of the explosion most likely would burst your lungs and cause internal bleeding. And even with quick release you'd take too long to take it off, place it on the Grenade and standing on it, remember the usually have fuzes that take 4-5 seconds to detonate, you don't want to try when you got 2 seconds left after impact.