Business casual: A v-neck sweater vest is a great piece for a business casual look. Try layering a pullover sweater vest over a button down cotton shirt in a complimentary color. For example, a blue shirt with a cream colored pin stripe would look nice with a light yellow sweater vest. A sweater vest can also add visual interest to a basic button down and sport jacket look by adding an extra layer. For example, you can layer a cream colored cotton button down with an olive green sweater vest and a medium brown wool blazer for a nice earth-toned look.
Relaxed date night: A vest is a great way to add extra polish to a date look while still appearing relaxed and somewhat casual. Try out this look for a movie date: Layer a pullover burgundy sweater vest over a red and gray plaid shirt, with dark gray denim pants and a nice gray loafer. You'll look put together and appropriate without looking particularly "dressy." For a look that is a little more appropriate for night time, such as drinks out at a bar, try dark wash jeans and a gray cotton button down with a black tuxedo vest.
Outdoor Wedding: I always recommend wearing a three piece suit with a vest to an outdoor wedding. This way, if you get hot, you can take off your blazer and still look polished. If you are wearing a dark colored suit, avoid wearing a plain white button down underneath (as this look is often favored by catering and restaurant staff). Instead, opt for a pin stripe or windowpane patterned shirt, or a shirt in a light or bright solid color. A gray three-piece suit with a white pin stripe is a nice look for an outdoor wedding. This can be paired with a white oxford shirt and a blue diagonally striped tie for a formal look that still reflects the lightness of an outdoor affair. You can, of course, experiment with different tie styles. A plain black skinny tie can also be a sophisticated look when paired with a pin-striped vest.
Artsy Event: For an event such as an art show or a gallery opening, snatch that gray pin striped vest from your three-piece suit and layer it over some other pieces. For a monotone look that still has great interest, select a pair of dark wash jeans, black fashion sneakers (such as Diesel or Pumas), a gray button down shirt, and a dark silver or gray tie. If the event is more casual, go for the same jeans with a classic Converse One Star shoe and a silk screened tee shirt. Or, try the pin striped vest with a plain white v-neck tee shirt and a pair of suede loafers or oxfords. The vest is that integral piece that will pull your whole look together.