A vest (known in the UK as a waistcoat) is a sleeveless garment covering the upper body. The term has different meanings around the world.
For purposes of tagging, vest is used for when the garment is/can be opened in the front, more traditional formalwear, or sleeveless armor
For garments that are single piece, generally more tight fitting, and more casual ware see tank_top See also https://en.wikipedia.org/wiki/Vest