Vents are usually the slits at the lower edge of the sweater or jacket similar to those shown in the picture. I suspect thare was a point after knitting about 5-6 inches in the back and fronts where you increased stitches. This notch is the vent. The long edge is at the ends of the rows. It'll give a nice finishing touch to the bottom of the sweater if you pick up stitches along this edge but you can probably leave it out. See how it looks when you pin everything together and then decide.