The vest is what completes the mystique of the three-piece suit, which is why you must pay close attention to its fit. A vest that is too loose will not hold your tie in place, and will shift around too much underneath your jacket. A vest that is too tight will restrict your breathing.
To test the fit of your vest, wear it standing up and sitting down. Don’t be afraid to take your time and ask your tailor to readjust the vest several times. An ill-fitting vest will ruin a three-piece regardless of how sharp it looks.