Body armor doesn’t turn someone into Superman.
What it does is stop a bullet from entering the body, or absorb most of the energy before it does so.
Getting hit with a bullet to a chest plate, especially a rifle round, will hurt you. It may even bring you to your knees. It’s like taking a hit to the chest with a hammer. It will hurt.
Did the shooter have soft or hard body armor?
Most rifle round will punch a hole right through a bullet proof vest.