A waistcoat isn’t used very often today, and its popularity has been descending. But you can’t find anything more classical than a three piece suit. If you want to avoid the classical look you can style your waistcoat with chinos and a printed shirt. It gives a modern look and can be used at almost every event or occasion.
Waistcoat button etiquette
If you choose to wear a waistcoat the same rules for buttoning the blazer apply. You should keep the bottom button unbuttoned. This rule only applies for single-breasted waistcoats.