I don't have any experience with training with a weighted vest, but am interested in learning what you find out. I'm just not sure adding additional weight to my frame and doing hill repeats would be too good on my joints and muscles. I wonder because while I haven't used a weight vest, I have been doing a TON of extremely steep hill repeats; i.e. power line cuts, XC etc... to the tune of 15-22k feet of elevation gain per week (training for Barkley Marathons). While I've trained with a pack with about the weight I expect to wear in the race (on occasion, typically on my longer outings) I don't think I'd like a weight vest unless it was just another way to get used to carrying the same load. How much weight does one typically add? And does the supposed benefit outweigh (no pun intended) the risk of joint damage say versus just doing additional hill repeats w/o the vest? In my case it made sense to just have elevation gain/week goals but if there was a benefit in substituting some of these workouts with fewer repeats but with a weighted vest I'd be interested.
It's no different than weight training and lifting. You add little by little to increase the load therefore strengthening your muscles. I can be beneficial to those losing weight while running because as they get stronger, they are also losing weight. So you can wear a vest to equal weight loss. I have used weights for quicker hands (swinging), more arm strength, and better first steps. Much the way you should train to run a 10k to be competitive in a 5k, I would train with extra weight over what you expect a pack to be. If you become accustomed to running with 50lbs, when you add a 20lbs pack it will feel like nothing.