If their head armor is weak, strike the head, if there boots are low quality, strike for the feet. Naturally, a head strike is more difficult than one to the body. As was mentioned above, AC had a simplistic technique for this with a high, medium, or low strike though the actual locations hit were randomly determined. Ultimately, what this pieced armor technique tried to capture is the idea that striking vulnerable areas can lead to more damaging attacks if successful. Rather than doing this through pieced armor, there is a simpler technique which may give more realistic results. Why not allow players to have a sliding scale to toggle between "to hit" and "to crit?"