IMHO and Hubbies - Leech lines are the devils work. A leech that is vibrating is showing that air is coming off it well, its the differential pressure that causes the flutter. The differential pressure is the reason sails work, as in aerofoils.
Tightening a leech line and stopping the flutter can hook the leech and decrease the performance of the sail. Compromise is to tighten just enough to decrease the vibration but not enough to hook the edge over.