For backpacking, I wouldn't buy a vest for "storage" unless it had more than one function, such as also providing insulation. The vests I've seen are made of canvas so provide little if any warmth. I'd also be worried that wearing a vest with lots of lumpy items in it under your backpack will be very uncomfortable due to the bumps under your pack harness. Ouch! Besides, you'll have to take your pack off to access the vest.

Several companies, including ZPacks and Mountain Laurel Designs, make very lightweight belt pouches that will take care of extra accessible storage and weigh a lot less than a vest full of pockets. You can access a belt pocket without removing your pack. I have a detachable belt pocket from ZPacks that I use to carry my essential fishing tackle. It weighs 0.75 ounces. For 1.5 ounces you can get two of them that can be used with or without your pack. ZPacks also has a larger size pocket that weighs 2.9 oz. It can be used as a pack lid, a chest pack, a fanny pack or an over-the-shoulder satchel and will probably hold as much as or more than a 16 oz. canvas fishing vest. The price is a lot less, too! I also have one of their shoulder strap pockets that I use for my camera.

I have both these pockets (I got the big one for my grandson). My only relationship to ZPacks is as a very satisfied customer.

This all assumes that, while you are moving, you really, truly need to access more items than will fit in the belt and/or shoulder strap pockets that come with most packs.