Embrace the double vent.
There are a number of questions that your tailor will ask you and, for the vast majority of them, there is no right answer. Not so when it comes to vents, the slit or slits up the back of your jacket. Spare yourself a lot of grief and tell your suit maker that you want deep double vents. You'll have an easier time sitting down since you won't be sitting on the back of your coat. The suit will hold your silhouette when you walk because it will give way to your legs as they move. Vents are, quite simply, not the place to take risks.