The force is still transmitted to the target, even though the projectile may not penetrate. The result could be considerable — even lethal — blunt-force trauma. Still better than having your innards pureed by a bullet, I suppose.
A friend of mine who was an ATM tech was shot center of mass twice with .38 plain lead round nose loads. His vest stopped them but it knocked the wind out of him. He felt lucky that the thieves didn’t finish him with a head shot. To add insult to injury, the FBI visited him later. They suspected that it was an inside job because the round used was virtually guaranteed to not be lethal under these circumstances.