The vent pull down conical parachute offers some of the highest drag out of any parachute. The vent of the parachute is pulled down which relieves forces in the crown and allows the canopy to inflate to a larger diameter. Larger diameter=more drag. This techniques is used in the military (G11's) and in skydiving (pilot parachute) and is the easiest way to gain drag without modifying the parachute's shape. When you compare the VPD's shape to competitors you can see the difference immediately. The vent band (the reinforcement sewn inside the top seam) is loaded and forms a symmetrical shape. When you look at competitor's loose vent band you have to ask, "what part of the canopy is seeing all that force and is the canopy going to fail?"
How to size: To remain competitive without making things confusing, use a drag coeffient of 1.05 on the calculator page.