The air vent should be over the point where the water heads down the drain... the physics of a durso is rather complex, and there is not really a siphon going on, so one would think this would not make a difference, however, the system will probably start sucking air too soon, at a lower flow rate, and sucking in air is undesirable with an open channel drain. Plus that pipe is awful small to be tyring to run it open channel smallest practical durso is 1.25". For a marine system, you are going to need an overflow in front of that either way...for surface skimming/surface renewal/gas exchange, and disolved organic processing by a "skimmer"