A lot of variables. Distance from Muzzle, Muzzle Velocity, the build of the individual wearing the vest, Rating of Vest etc.
That said, I would expect that if a .357 158 grain JSP strikes center mass on a NIJ Level II/HG1 or higher vest that is in good condition from about 5 meters or more, that the round will not penetrate based on the NIJ test protocols. Based on comments from gun shot survivors and the news, I would expect bruising and discomfort.
HOWEVER, if the round is a special threat round or a contact shot, I am not confident in the above. FBI and DEA have their own armor standards that specifically require contact shots. While I am not an expert on Special threat (High Velocity, AP, etc.) rounds, nor am I tracking a 158 GR .357 one, but.....
A factor in what the shootee will do when hit is mental. If they have been watching movies and have been conditioned that when people get shot they fall down, they might throw themselves on the ground. If they watch Mythbusters and know that a gunshot won't necessarily throw you around they will probably stand and fight.