height of the vent won't affect when the siphon starts, as long as it's not higher than your 10" pipe.
Now this confuses me. I am like Gully washer, in that I have no experience with a siphon, but I have read a lot about them because I want to install one someday.
The height of the vent determines when the siphon is turned on and when its turned off. If it was below the outflow pipe, then the outflow pipe determines when it turns on, but the vent would determine when it is turned off. Also if the vent is below the outflow pipe, I would think that there may be more of a chance to have trapped air. .... Am I wrong about this?
My thought in the holes would be a trash guard of sorts. Not for water being blocked from getting in, but for an insurance that the siphon mode will break seal and not drain the whole pond out.
Mike-- does your inlet of the siphon go down deep into you pond? Or is the inlet just at water normal water level?
http://www.ponddampiping.com/syphon1.html 
