Good point and, in fact, Kevlar alone isn't necessarily effective against a lot of firearms and even against knives:
[S]oft body armour vests are ineffective against most military rifle rounds. The CRISAT NATO (Collaborative Research Into Small Arms Technology-North Atlantic Treaty Organization) standard for body armour specifies the use of titanium backing. This titanium plate may be removable or sewn in.
A vest does not protect the wearer by deflecting bullets. Instead, the layers of material catch the bullet and spread its force over a larger portion of the body, absorbing energy more quickly and hopefully bringing it to a stop before it can penetrate into the body. This tends to deform the bullet, further reducing its ability to penetrate. While a vest can prevent bullet wounds, the wearer still absorbs the bullet's energy, which can cause blunt force trauma. The majority of users experience only bruising, but impacts can still cause severe internal injuries.
Most vests offer little protection against arrows, ice picks, stabbing knife blows, bullets with their points sharpened or armour-piercing rounds. As the force is concentrated in a relatively small area with bladed weapons and armour-piercing rounds, they can push through the weave of most bullet-resistant fabrics. Specially-designed vests which protect against bladed weapons and sharp objects are often used in vests for prison guards and other law enforcement officers. Some materials like Dyneema offer considerable protection against bladed weapons and slash attacks.
Most ballistic vests may provide little or no protection against rifle ammunition or even against handgun ammunition fired from a pistol-caliber carbine. However, vests of type III and up are built to be resistant to rifle and armour piercing rifle fire, the exception being .22 LR ammunition, which can usually be stopped by these vests even when fired from a rifle. These vests are usually protective against handgun ammunition fired from handguns of all calibers, depending on the armour level of the vest.
