Wearing a vest with a 3b suit is a strange idea: if he buttons the suit the vest will disappear.
Not really. The stane of the vest just needs to be high enough. Besides, I think only the top button needs to be seen, so, not much higher. There are some three-buttoned suits with a high button stance (perhaps even as high as a four-buttoned one) and the stance of the vest will have to compensate, resulting probably in a very high-stance vest. If the vest happens to come not with the suit, then the best thing is to unbutton the suit jacket. Maybe it is just me, but I like unbuttoned suit jackets when vests are worn (because it shows the vest), and don't quite like seeing vests hidden under the suit jacket (when buttoned). Either all or none?
: Oh wait, how about this odd combination? Black/Dark suit with white vest (and a light-purple or pink shirt)? Perhaps it might be tasteless when executed, but it looks rather good when I think about it. WJTW