If the vent is mounted near the highest point of the pipe, as the mainline fills, air will be pushed out the vent tube as long as the pressure at the open end isn't greater than it is on the inside opening...following the path of least resistance. Once the downhill side is completely filled with enough water on the downside that the weight of is greater than the weight of water it has to lift on the intake side, the siphoning starts, some remaining air left in the vent will be sucked back into the pipe (If the air volume inside the vent pipe is too much than it could potentially defeat the siphon) Assuming the vent is appropriately sized, during the siphoning, water should be getting pulled up through the vent. It will be part of the intake. The water filling the outlet pipe works like a plunger sliding down the pipe pulling water up the intake side. Once the water level drops below the vent intake, it will be easier to suck in air than water (again if the air vent is properly sized) The length and angle of the pipe on outlet side are going to have a major impact on the power and effectiveness of the system. The steeper and longer the downside of the pipe the more suction will be developed.
