I don't like bottom style drains. To do them, you have to damage the integrity of the bottom of the dam. A siphon will do the same thing without any potential problems.
Also, it's much easier to add another drain pipe, or repair the one you have if needed. Keep the vent hole caged or protected because creatures of the night will get in them, and trash will accumulate and could potentially keep the vacuum from breaking. I had a huge turtle stuck in the vent once, and dropped the pond almost a foot below pool before I got him out.
