Here is a video of a man shooting himself in the chest with a 44 magnum. He follows it up by shooting a man in the chest with a 308 WIN rifle who happens to be standing on one leg. Neither man moves a muscle (they do have some serious faith in the products though). I laugh when I hear knockdown power. If a 44 magnum and a 308 WIN can't "knock down" a man, a 9mm or 40SW sure can't. Heck, even a 45 ACP might not be able to do it.
The man was standing there knowing the shot was coming, and that type of vest spreads the impact over a wide area, and with padding underneath. The vest did what it was supposed to do in absorbing the impact.
To knock him down under those conditions you'd have to tackle him. You still seem to be seeing knockdown power as force applied over a wide area.
The reporter in the video I mentioned was wearing soft armor, no balistic plates or padding other than the kevlar itself. The bullet did not penetrate but its energy was transmitted to a much smaller area, with the result of him ending up on his backside.
It takes only 14 foot pounds of energy applied to the outside of the knee to put an average man down. It doesn't take much more than that applied from the front to dislocate or fracture the hip, yet each leg can support 300 pounds or more and will hold up to exerting 300 pounds of thrust far longer than any human could apply it.
A bullet that can transmit enough shock to a joint or vertabrae will put a man down, no matter who he is. A bullet that can't transmit that force with the same point of impact won't put a man down.
A bullet that can burrow into a bone or pass through it without breaking the bone is less likely to put a man down.
Its not purely a matter of ME, some rounds with the same or higher ME than a slower heavier bullet are less likely to put a man down.
As for recoil.
I've seen people get knocked on their backside the first time they fired a shotgun with no idea how hard it would kick. After that first time they never get off balance from recoil again.
No handgun recoil is going to apply force directly to the body, the hand and arm absorb that recoil.