The only benefit that the vacuum bags have over the steel ammo cans is that the container itself doesn't weigh anything and if the ammo cans didn't fit the way you wanted in a larger container or something.

I might vac-pac a couple of bricks of .22LR. I'm in a humid climate, so what little air remains will have a little moisture. Should I put something else in the bag as well, and if so, what? Would a spoonful of uncooked rice help?