Wearing a vest all day is a recipe for orthopedic disasters. They will wreak havoc on your postural muscles and lead to postural distortions such as the dreaded upper-crossed syndrome that will eventually lead to severe pain syndromes and biomechanical inefficiencies up and down the kinetic chain. I have read about old timers wearing the vests for a week at a time and I believe it was steve justa who wrote about wearing one all day at work for a month. They got decent results, but not great results.
Weight vests are best utilized for enhancing bodyweight exercises such as burpees, pushups, pullups, ring exercises etc... Wearing them for short intense bouts of enhanced body weight training yeilds great results. You dont have to limit it to bodyweight exercises, as they work very well with many kettlebell movements and to make things like farmers walks and clubbell swinging more difficult, by all means be creative, but keep in mind that extended use will be detrimental to your musculo-skeletal system