Suit by definition means three pieces that originally siginificava jacket, pants and vest. With the disuse of waistcoats for men adopted the denomination Costume (why not Duke?), But we usually call Suit. Wrong or not, the power of the word to suit remained the double jacket and pants to the present day. Sale without Suit Vest or Suit with Vest. Moreover, it is very hard to find custom suits with vest to sell in stores dedicated to men's clothing. Pictured actor Michael Fassbender literally wears a suit.
If we consider that vest is not mandatory in costumes social, but it is essential that the tie is formed around the new trio to end so many doubts and controversies. The vest is ideal for cold days and gives more elegance to formal attire. The ancients were made of shiny fabrics, brocades and even embroidery and were adopted in England by 1660. Then become discrete when evolved into the very fabric of jackets creating the concept of Suit. Actually the vests are from Ancient Rome when they were made of iron to protect soldiers in battle.
The word arose from the Costume Costumes for Women and siginifica tailoring and sale of costumes handmade bespoke. Custom order or "custom". However accustomed to call Costume even custom made suits industrialized.