I beg to differ. Your experience appears to be with military body armor, which is very much different from that worn by U.S. uniformed police officers (the stuff worn by tactical/SWAT types is similar to the military versions). Military body armor is usually worn as the outermost layer, while police body armor is usually worn concealed (for some reason, a lot of Canadian officers wear it on the outside). The ballistic ("bulletproof") fibers in both types are typically Kevlar, Spectra fiber, or Zylon. The latter was recently found to degrade in terms of strength in a couple of years, and a couple of vests made with it failed to stop a bullet that they was rated to shield against (this is the first time that this has happened since soft body armor was introduced in the early 70s). Zylon vests are now being recalled and replaced, and the fiber has been withdrawn from this use.

Zylon vests are much thinner, lighter and more flexible than Kevlar or Spectra, but they're still fairly bulky. It's like wearing several layers of heavy cardboard under your shirt. Unless your upper garments are fairly loose or bulky, it's pretty easy to tell if someone is wearing soft body armor.

The trauma plates do function as I described, and that's not entirely inconsistent with what shillard said. The trauma plates spread the impact over a larger area to minimize injury. They do add some extra protection against higher-velocity rounds like those from rifles, but a rifle bullet to just about any other portion of the vest is potentially fatal to the wearer. The vests are only rated to stop projectiles that will be stopped by the soft Kevlar/Spectra layers.

Composition of the trauma plates vary with the manufacturer. The early models were all metal, but some had a tendency to deflect a round into the wearer's neck and head. Some are still made of metal, but with a different design, some are layered as metal/ceramic/ballistic fiber, and some are all ceramic. As with so many other things, which one is best depends on which salesman you are talking to.

Military body armor is not widely available for sale. Police-type body armor is available from any number of police suppliers, like Gall's and Quartermaster.

Someone made a facetious comment about other bulletproof garments. In fact, there are ballistic groin protectors. They are triangular-shaped pads of ballistic fiber that are stuffed in the front of the pants. I knew several officers that bought these, but all have up wearing them quickly because they were so uncomfortable.

And, so as not to hijack the thread more than it already has been, this will be my last word on the topic.