Springfield Trapdoor rifles will fire a 405g minie ball at around 1,350 fps, with 1,638 ft/lbs or energy. Compare that to a 1oz (437g) 12 gauge slug at around 1,550 fps, and 2,331 ft/lbs of energy. A level IIIA vest is rated to stop a 12 gauge slug, so it isn’t surprising it stopped the 500g minie ball which would be going slower that 1,350 fps (unless it was a hot load).
In vest penetration, it isn’t projectile size or energy but rather velocity that penetrates. 5.56 NATO - a 55g .22 caliber bullet at 3,250 fps (1290 fpe), will zip right through a IIIA vest. Surprisingly, a 20g 17HMR round at 2,375 fps (250 fpe) will also zip right through a IIIA vest. Around 2,100 fps seems to be the velocity at which a projectile will defeat soft body armor. Projectile design and composition also are a factor, but not as much as velocity.
As noted above, blunt trauma is a bitch. A vest may stop a heavy projectile, but you are going to feel it. If it hits the right place it could be lethal.