The Kevlar Helmet is a piece of equipment obtained at Level 15
It is designed to protect you from instant death via headshot.

Headshots are achieved by successfully hitting the enemy's head if a headshot is pulled off it will roughly double the damage of that shot. For example, if you hit an enemy with a 40 dmg shot it would become a ~80 damage shot.

With this, helmets also have a health bar that acts as armour which is deducted first before your health. With countless amounts of testing, we have concluded that the helmet has an AP (armour value) of 100. Meaning if you hit by a 55 damage shot it would only take off 10 HP (tested with the m4). (Damage of weapon*2)-AP value(Initially 100)

If you got hit with a 40 damage shot it would negate all damage and you would still have 20 armour points left on the helmet (tested with the makarov). If you got shot again with the same damage you would only take 60 damage instead of the usual 80 because you had 20 armour points left from the previous shot.

We tried headshotting with a 150 damage (AWP, sniper rifle) gun with a helmet on. As expected it depleted all helmet hp and all player hp as 150 x 2 = 300 and 100 + 100 = 200 which means the sniper bullet went over by 100 damage thus killing the player impacted by the shot.

Once all 100 armour points have been used/depleted the helmet will be useless and do nothing until your next life.

(You can tell if you fully negate a shot if it sparks and you hear a 'tink' noise)

This section will explain how the Armor Vest works, note that this section will be shorter as this guide has already explained a lot of the base mechanics of armour in the Kevlar Helmet section.

We put the Armor Vest to the test as it is achieved at level 16, 1 level higher than the Kevlar Helmet.

First of all, the Armor Vest only protects the body region of the player model and not the legs or head.
In comparison to the helmet that is a way larger region than the helmet protects.

We tested different damage values, first with the Makarov (40 Damage). The armour, without surprise, deflected the bullet. We shot again, this time it did not but it only took off 20 base player HP leaving us with the conclusion of the Armor Vest has an armour value of 60. This happens to be 40 lower than the helmet despite the helmet being a lower levelled item, why?

Well, this could be because of the fact that headshots double damage and shots to the body do not. If damage was not doubled during headshots (50 x 2 = 100 / 2 = 50) the helmet armour value would be reduced back down to a 50 armour value. This means that the Armor Vest's armour value is lower simply because body shots do not double as headshots do.