I don't have any experience with a siphon system, but, I think I understand the concept. The siphon action won't start until all of the air has been displaced in the pipe by the rising of the pond's water level. Or, you can manually fill the pipe with water, while bleeding the air at it's high point. Then, open block valves at each end, to begin the siphon. The purpose of the vent is to automatically break the siphon when the surface water has receded to a desired level. At this level the open end of the vent pipe is exposed, which allows air to enter the system. If not for the vent, the pond would drain until the water level dropped far enough to expose the intake end of your overflow pipe. If you lower your vent, you will only delay the breaking of the siphon. It won't hasten the beginning of the siphon.
As to the question of why the engineers suggest placing the end of the vent pipe below the water's surface, and drilling holes in it; I'm guessing, maybe it's a way to prevent a piece of floating debris from be sucked up into it, and clogging it.
Just a thought, but, if you did install a second siphon, like you mentioned in your other thread, maybe you could place the opening of the intake at desired water level, and draw water off the top, thereby eliminating the need for a vent. The siphon will be broken when the water level drops, exposing the top of the pipe. Of course, you would need a trash guard.
I'm sure you already knew much of this. Just my way of trying to explain. I find it challenging to discuss topics like this in a few words.
