Yep, holes are for a trash gaurd. And as already mentioned, height of the vent won't affect when the siphon starts, as long as it's not higher than your 10" pipe.
When the siphon does start, what air is trapped inside the vent pipe will get sucked through and then for as long as the main pipe is in siphon mode, water will also flow through the vent pipe. "Flow" is too gentle a word, though, it's actually pretty violent. Thats why it's a good idea to anchor your vent pipe.
