Excluding exceptions, the vest must always be worn over a shirt with a buttoned collar and under a single-breasted jacket or blazer. It should never be worn under a double-breasted jacket: the neck of the jacket, in fact, must be large enough to allow the waistcoat to be visible.
It must be snug and adapt perfectly to your body, so as to make it comfortable to carry the jacket over it. Its length must be sufficient to be able to cover and hide the waist on the front, while laterally and on the back it is usually shorter.
For these reasons, opting for a made to measure vest is always the best solution: you will be sure to wear a vest of the right length, made to fall perfectly and adapt to your physicality.
How to match the vest to the dress
When it comes to choosing the color and to match the vest to the suit, there are mainly two color alternatives available: opting for the three-piece suit or choosing a contrasting vest.
The vest should always be worn with a shirt with a buttoned collar; this also requires wearing a tie, to underline the formal cut of the suit.
The bow tie is less advisable, which stylistically does not suit the three-piece suits. Unless you want to amaze with a detail over the top, it is better to always prefer the tie.
If you really are in love with the contemporary dandy style, you can also leave your neck free and give up the tie, provided you choose a context and a clearly informal outfit. Remember that freedoms of this kind are banned on the most elegant occasions.