A fair number of the items in my GHB are vacuum-sealed. Dry food, food powder mixes, bouillon cubes, etc; to keep out both water and oxygen. Even my change of clothes in there is vacuum sealed, to keep out moisture & bugs. The 35mm film canisters of cotton-ball & vaseline "tinder" are vacuum-sealed as well, largely to protect the rest of the pack from them if they were to open & leak.
If stuff in your pack is vacuum-sealed, it’s a good idea to keep some empty Ziploc bags in there as well. Once the seal is broken on the vacuum bag, I’d hate to be without a way to re-seal items. I keep several of the gallon & quart “slide” zipper type bags in my pack, and most of the stuff that’s vacuum sealed has a Ziploc bag or two just sealed in right along with the ‘stuff’.
Vacuum-sealing is also good for relatively long-term storage of some items. Powdered chlorine, (Sock-it), for 'making' liquid chlorine bleach for water purification; vacuum-seal the chlorine powder package, along with instructions for usage, and it will last a long time. Normal liquid chlorine bleach has a surprisingly short shelf life. It gets weak real quick; so the more stable powder form (especially vacuum-sealed in its original package) lets you keep it a LONG time, and takes up almost no space. I calculated it out one time, and iirc, just one bag of the sock-it I've got put back is enough to make enough liquid bleach 'stock' (I forget the number of gallons) to treat 5 gallons of water a day, for something like 26 or 28 years straight. Cheap, easy insurance for something as critical as water purification.
