If the bullet is stopped, the inertia of the bullet as it slows/stops will still be fully transferred to your body. This could cause serious injury to bones and tissue, not to mention the possibility of severe internal bleeding if any organs erupt due to the force.
I don't think you transfer 'inertia'. But this sounds like an energy transfer argument as compared to a penetration and destruction of tissue argument.
Vests are designed to spread the energy, esp. with pistol rounds. Rifle rounds are a different beast. If it were just the energy - your hand would erupt from the force.
Energy transfer as a crucial component of stopping power isn't that much of a factor with handgun rounds. If a home invader come in with a rifle - you are screwed with the common vest - which you aren't going to be wearing as you watch COPS.