A low velocity handgun bullet's pattern of energy dispersion will usually
depend on the thickness of the Kevlar and the nature of its weave. Thinner
armor will leave a central dark bruise with lighter bruising over a diameter
of a few inches. The central dark portion is because of the natural flex in
the aramid fiber with more direct energy transmitted directly under the
bullet impact point. As the armor gets thicker, the central area of bruising
gets lighter as the energy is dispersed over a broader area and there is
less flex in the armor. The weave of the armor also plays a role in its
energy dispersion. If the armor has a central ("10-ring") ceramic trauma
plate, the energy is dispersed broadly enough that there may be only uniform
bruising. In many cases, there is sufficient energy transmitted to a small
enough area from the bullet that it may be enough to fracture an underlying
bone, such as a rib or sternum. It's all related to the energy in the
bullet, which is entirely related to the mass of the bullet and the velocity
of it. Obviously, the closer the shot = higher velocity = greater energy =
more bruising. The extent of  the characteristic dark central area of
bruising will be less as the armor gets thicker, deforms less, and spread
the impact energy over a broader area. Note that often, the energy is
sufficient to actually create a wound beneath the impact point from the
crush damage.

These are issues I've had to occasionally deal with as Assistant Coroner and
death investigator for this county and as a surgeon who deals with a variety
of blunt trauma injuries. A rather scholarly treatise on the subject can be
found at http://sipi.usc.edu/~kosko/NAFIPS-04.J04.pdf .

HMc