In order to flatter the upper body, a waistcoat must be quite short, and therefore the trousers high enough to overlap by two fingers’ breadth. When buttoned, the jacket must leave a couple of waistcoat buttons visible, but a dinner jacket must cover the waistcoat completely. In effect, a waistcoat sets the tone for the whole suit.
A formal waistcoat generally has six buttons and four pockets, which are not only useful for putting things into, but also for a range of gestures to enable the wearer to say many things for which either no words exist, or for when it would be better not to say them. To express annoyance, haste, superiority or even contempt, you only need to put your index finger into the higher pocket on the same side and move the other fingers nervously.
The lower pockets are ideal for your pocket watch, which, even if broken, is a perfect regulator not necessarily of time in itself, but of how much time we want to give others. Just take your watch out to consult it, perhaps raising an eyebrow, giving the other person no other option: the conversation has become less interesting and now it is time to go.