Not yet in Meso 2 - doing Meso 1 on the six month plan - but ... I've needed my weighted vest for legwork from the start of Meso 1, for squats, and to help reduce the number of dumbbells I had to run out and buy!
It's also been very helpful in letting my forearms/upper body build up to the heavier weights that the Legs workouts require. It seems to work well to start with more of the weight in the vest, and then move it gradually to the dumbbell.
Also, just in general, I'm finding that I greatly prefer using the vest for lunges to dumbbells alone. It seems to put less stress on my elbow and shoulder joints; at any rate, there is less of tug forward as I move forward. Anyone else noticed that? Perhaps I have a form problem - a checkup with a trainer may be a good idea - at any rate, shifting some weight to my vest from my arms really helped combat the bad stiffness in those joints that I sometimes used to get. And wearing the vest definitely increases my calorie burn during the workout (I leave it on for most of Legs, except deadlifts, and have started wearing it loaded very lightly during Triceps, except for deadlifts and pullups).
Side note, I worked up to a fully loaded vest before starting STS over several weeks, increasing both the weight and the length of time I would wear it, and am doing the same thing now that I've purchased one that goes to 40 pounds.