Yeah my bet would be making a “lethal force” combat mode and a “non-lethal force” combat mode.
Some weapons would have both of them (like swords). In that case, the non-lethal would deal less damage overall, but it wouldn’t kill the enemy.
Other weapons could have only one of the modes. A military flail cannot be used safely to knock out an oponent. On the other hand, a chair leg won’t outright kill an oponent. That would require a conscious execution once the opponent is KO.
This described above is the system used by D&D 3.5 (but not many people use it, since lethal force is required 99% of the usual combats).
As for armor… Yes, a sword isn’t that lethal against a guy in armor. But that’s because you’re not dealing any damage to him. To use RPG terms, the plate armor is absorbing damage, not mitigating it. Therefore once you get around the armor and hit where it cannot negate the damage (you put a dagger through the visor), it’s dealing only lethal damage. You hardly would ever get to do any non-lethal damage, since it would be mitigated twice, once by your containment, another by the armor.