Basically any pistol can defeat soft body armor with the right load. You would have to do tests on real body armor to find out, but I would think that any load that goes over 2000fps would be a danger to body armor. It's definitely possible to get 357 mag and 9mm going at that speed with fast powders and light bullets.
For extra penetrating ability, I would try solid copper slugs. That would be legal. If legality is not a concern, a steel penetrator is likely to defeat armor, even at lower velocities.
Armor works by allowing the soft lead slugs to deform. Steel isn't soft and doesn't deform.
Someone should make up some legal handloads and do some tests to prove how stupid it is to go after the Five-seveN. The 57 was designed to defeat military flak vest type things which are not the same as what cops wear.
Btw, all my comments above about how to defeat armor: I haven't actually tested any of this and no one publishes this sort of thing so it's all hypothetical for me. It's based on my understanding of what the soft armor is capable of and what it isn't, and the well-known fact that basically all rifle rounds will whiz through basically all soft body armor.
