Scientifically, I don't know (and don't really care lol), but I've been doing weighted vest walks for a little over two months now and I totally feel like I've built strength and muscular endurance. I'm currently using 50 pounds for a 2 mile walk right now.
When I was a young whipper snapper, we did runs in the Marines with a ruck on, which weighed 40 pound plus a load out and rifle-- so about 50-60 pounds. At the fire department rookie school we would have to jog with turnout gear and SCBA on (not as much weight, but easily 20-30 pounds.) Both, even at the time felt as good conditioning methods to train your body to work with the equipment and added weight on your body.
The weighted vest walking has been a great addition to my training, my traps get worked hard, I can totally feel my whole body supporting and moving the extra weight with every step-- plus, it just makes cardio/conditioning seem more hardcore when you have an added load involved. I'll throw on the vest, turn on an interesting podcast or something and go for a nice paced walk.
Running wise, I think it would kill my feet and knees from the pounding now, but for someone who is comfortable with it-- I think running with weight is definitely a great way to build strength.