Just think “dapper.” Suit vests (also known as waistcoats) are meant to go with a three-piece suit. Luckily, you do not need to wear these vests with a suit (much less a three-piece), ever.
These are your classic vests; they’re wearable just about anywhere in any style (buttoned, fitted, loose, fancy, casual). Best worn all the time. Should be paired with anything and everything. Wear with whatever you want, including collared shirts and T-shirts and jeans and converse and ANYTHING. (see me in mine.) Warning: very few people can pull of the wrinkled/stained shirt and vest thing (think Shane, amirite?); I like to use the vest to pull together an outfit that’s already crisp and clean, instead of making a sloppy (but totez hot for some people) outfit look better.