Generally, the vest on a 2-button suit should show above the top button of the jacket approximately 2 1/2 …just enough to see the vest, but not cover much of the tie. I’d recommend slightly less reveal if your suit is a 3-button. Perhaps 1 3/4″-2″. Remember, a 3-button suit will have a higher button stance and therefore a smaller lapel and “gorge”.
All suits, ready-wear and custom are designed theoretically to produce these results. Ready-wear suits may be assembled, however, with vests that don’t meet the most rigorous style tolerances…so check the stance of your vest if buying what is known as a 3-piece suit (jacket, vest, pant) off the rack.
Please send your sartorial questions to me at firstname.lastname@example.org.