"Does anyone know exactly how alpha vains work on the 737ng??"
I guess you're referring to the Angle Of Attack Sensor(s) on the side of the airplane (not far from the nose)?
I'm pretty sure these would work the same way they do on other aircraft. They are simply small wing-shaped or wedge-shaped devices which pivot up and down when the airflow around the fuselage is fast/strong enough to move them. They sense the angle of the fuselage with respect to the airstream/flow in which the aircraft is travelling.
There is an electrical transformer (hidden) inside the aircraft which senses the angle of each
vane (wing/wedge). This transformer converts the angle into an electrical signal which is then sent to the aircraft's air data computers. This AOA data is can be used for a variety of things (e.g. to help generate stall warnings).
Here's a shot of a 747-400 AOA sensor:
http://www.ozemail.com.au/~iriddell/AOASensorNew2.JPG
Please ignore the red line (it was added later to show the "neutral position")
"Also when switching the apu from off to start on the 737 ng, what triggers the switch to goto the on position? "
It's the same as an Airbus. It simply stays in the Start position as long as you hold it there. Otherwise is simply springs back to ON.
Rgds.
Q.