I to got to jump on the vest and back problem. I bought the Simms back brace (which is like the Orvis) but I bought it cause of trying to walk on the rocks, colder water, etc, I was tensing up and that was killing my back. Back brace, and walking staff cured that problem.
I use the Orvis Fanny pack/chest pack combo. I put all the boxes I need in the fanny pack (and that means about 8 system X) and put my tippet and tools in my chest pack. The fanny pack has a built in back brace like my Simms, plus cinch downs so it does flop up and down while walking or running.
I say it will hold that many boxes, but I of course maybe carry 4 at the most if I am not sure, then I have room fo extra reels a camera, munchies, plus two water bottles.
Then of course if wading deeper water, I wear the chest pack only.