I like to wear a three piece or odd vest with an open collar shirt now that I don't wear ties anymore >.>
Also my emergency shirt which is a tent like white hickey freeman shirt gets hidden away by the vest. And then the super lazy times when I wash my shirts but don't feel like ironing looks marginally better with it's wrinkles partially hidden away.
Here's a quick-pic sans tie. No tailoring done on the vest or pants yet.
I would totally rock this as a 3 piece. Granted not something you could wear to an interview or board meeting, but would make for a kick ass party suit.
Why don't I ever see people wearing three piece suits aside from on TV? If I had the money, I'd have 112 bespoke three piece suits, one every day for four weeks for every season.
Because a waistcoat is uncomfortable in warm weather and overheated buildings of today.