Consider that death is just one way to achieve your goal. Incapacitating your enemy is often the overarching goal! Though I'm sure that you can kill via blunt force trauma with bullets against future body armour, incapicitation is much closer. It is sometimes more effective too.
A body armour isn't as in the movies where they shrug off small arms fire. A hit with a run of the mill pistol can already leave a bruise bigger than an outstretched hand. Some future armour will probably protect them better, but your soldiers will have assault rifles and bigger. There will definitely be blunt force trauma. As with any blunt force trauma, many hits can take you down no problem. Boxers work on this principle, where damage to the body and preferably head will incapacitate the opponent over time.
On the battlefield many will still be able to continue to fight after a hit. They are filled with adrenaline, increasing pain thresholds and their will to continue. But now imagine a soldier hit, leaving a 20cm bruise. He might still be able to fight later, but it takes a big toll. If the soldier is hit multiple times or just badly the first time the person is likely to be useless after the adrenaline leaves the body. They need medical attention, draining resources and time. Depending on the amount and where they are hit, this can be a long time. Imagine this at a front with thousands of soldiers and it can become a logistical disaster. Technically you're better off with many dying instead of tying up huge amounts of resources for big hospitals and transport for medicine, extra food and much more. If not done correctly, this can take an army off the map thanks to the logistics problems. Dead people need to be burried or transported back, injured need to be properly taken care of.
Though the difficulty to kill directly is heightened,it is still a possibility. A few good hits and they are incapacitated, dying or dead. All are desirable outcomes in war.
Reducing the energy
Reducing the energy of a bullet is difficult. Newton's law tells us that every action has an equal opposite reaction. Simplified this means that the one firing the bullet experiences the exact same amount of energy as the ine being hit. The reason they are much better off is that the arms can move more easily to mitigate the impact, as well as the acceleration is just a bit longer than the deceleration against the body.
To reduce the impact you need to reduce the speed over a longer time, against things that can move more easily to take the impact. As it's hard to control where the impact will occur, we can mostly just add more layers. We can also try to spread the impact even further, but this might be difficult.
Because of this it is best to employ 2 tactics, which are often employed. First is not getting hit. That is why tactical advantages are so important. You want to maximise the ability to hit your opponent while minimising your own.
If you do get hit, deflect the bullet. Much like a helmet. You don't want to stop the bullet, you want it to impart as little energy as possible! Helmets, tanks and a lot more are all designed to do so. Thanks to oblique angles you either push the bullet on a different path, receiving little energy yourself, or the bullet has to plow through much more material as it's set at an oblique angle. Either way you have some advantage.