Steve,
Bottom line is that you want to create a convection airflow within the kettle to circulate the airflow over and around the food, this will impart the smoke flavours plus absorb and impart some of the cooked food flavours back into the food. The orientation of the lid vents will depend on the way you set-up the Kettle for indirect. For many indirect cooks I will put the fire 1n one half and the meat on the opposite side with the vents above and slightly to the left or right of the food.
Regards