I'm guessing stupid rather than bad aim - I'm guessing it was a pistol vest not rated for rifle rounds. Level III-A is not Level III. Someone stupid enough to do this has to be stupid enough not to read the rating on the vest.
They come with Instructions?
He had a vest and a plate. Moat vests are IIIA and plates are III. IIIA is rated for handgun rounds up to .44 mag, and III is rated to stop an AK round or 223. Your mileage may vary.