Starting simply, you can throw on a vest on pretty much any long sleeve shirt or sweater. The Penfield styling example (pictured left) racks up some extra points for the elbow patches. If I catch you wearing a down vest over a short sleeve anything I will be very upset with you, it just makes no sense.
The over and under: for chillier days when a vest by itself isn't cutting it you can combine it with another piece of outerwear. Although this technique takes a bit more skill the payoff can be great if well executed. Aside from color coordination considerations the overall bulk of your layers is what will keep you from looking like Santa Claus in the middle of Fall. Both styling examples from Hayashi & Sons and The Bureau Belfast use thin jackets to make the look work. -Bob