King Charles II introduced the waistcoat as a part of correct dress during the Restoration of the British Monarchy. Samuel Pepys, the diarist, wrote in 7th October 1666 that "the King had yesterday in council declared his resolution of setting a fashion for clothes which he will never alter. It will be a vest." This royal proclamation is the first mention of a waistcoat. As you can see in the pictures you can wear it as casual as you like. Check also the blog of Street Etiquete.