If a cannonball was shot at you while you were wearing a bulletproof vest, it would tear right through your body instantly. Even with 20 vests stacked together, it would hit you with almost the same impact, plowing right through. At 100 vests, some energy would be absorbed, but the force of impact would still break your bones and crush your organs.
Now, if you were to stack 5,000 vests, creating a wall dozens of feet thick, you would finally stop the cannonball. But the force would still travel through the vests, and since they're attached to you, you'd still be thrown to the ground.
