It depends on how far away, and what you were shot with... vests are designed to block hand guns, not rifels. It may not stop every bullet. A larger hand gun, say 50cal, has more recoil and is harder to aim with sequential shots, thus not practical... but feels amazing and is so loud!

I believe it would take a person down and possibly kill them. I've been told police are taught 2 to the chest 1 to the head.

From my quick googling, a great read: "...Among the accounts from those who have experienced being shot while wearing a bulletproof vest (and lived to talk about it), there is one consistent analogy to describe the feeling: being hit with a baseball bat. ..." http://www.ponderweasel.com/shot-wearing-a-bulletproof-vest/