A unsleeved coat started out being called a Jerkin during the Buccaneer Pirate Era, they were short in length. During the Golden Age of Pirate, they became to be called Waistcoats, n be long in length. They were first very long to the mid calf length n' became shorter to mid thigh length. During the Late Pirate Era, the shorten and became Weskit. In modern times we call it a Vest.
King Charles II of England, Scotland and Ireland introduced the waistcoat as a part of correct dress during the Restoration of the British monarchy. Most of the time it is too hot to wear a Frock Coat, so just go with a waistcoat, as it is much cooler, cheaper n' ye can sword fight with one on. Button up the waistcoat when wearing a Frock Coat, but you may leave them unbuttoned when wearing them without a coat.