Casual vest are more about clashing than matching and should seek to appear quickly put together but not sloppy. Solid earth colors are easier to work with as they match most other types of shirts and trousers. More pointedly, they complement jeans better than bright slacks.
The vest is commonly worn with dress shirts and certainly will still work with a tie. That is way too stuffy for casual occasions. A solid dress t-shirt or a polo shirt with a earth tone or neutral color like white will work well in balancing the casual with something a step up from jeans and a t-shirt. Your outfit will be appropriate for many different occasions.
The type of events you would wear vests are in dining situations, casual social occasions our out running errands. It is not designed for formal events or lounging around the home. It does work well in most other situations except those that require professional clothing such as work.
Rule 1: Vests should be a little snug around the abdomen with large arm holes for comfort. The snugness ensures that you are sliming the body.
Rule 2: The vest and the shirt should be different but complementary colors.
Rule 3: A patterned vest should be worn with a solid shirt or a solid vest can be worn with a patterned shirt.
Rule 4: A plain T-shirt is highly casual while a polo rests in middle ground. A dress shirt can be worn if tucked into jeans or slacks.
Rule 5: Close your vest with dress shirts and open your vest with casual clothes.
Rule 6: A complementary derby hat can make a 1930’s theme out of it.
Rule 7: Make sure the shirt you wear doesn’t stick out under the vest and below the waistline or you will appear sloppy.
A few other tips in the video: