This is a discussion on Self starting siphon within the Beginner Saltwater Aquariums forums, part of the Saltwater Fish and Coral Reef Tanks category; -->
What is the purpose of the vent? They are quite noisy and I capped the vent on the one I built; it runs faster.
The air vent is there to prevent possible siphon break.
As far as the placement of the exit as long as t is lower than the intake,the only time I have used this model I had made it as close to the original design as possible.
I presume if you changed its position it would change flow rates and one would want the maximum flow rate.