Okay, as a gift (for carpooling a co-worker when he lost his licence) I was given a vacuum sealer.
It's good for sealing up stuff for the freezer, and seems to help it stay fresher for longer in there.

But what else is it good for that a ziplock wouldn't be better for? A ziplock can keep water out, is easier to open and easily resealed.

I have this nagging feeling that it's good for more than just its' intended purpose. Like, ammo?

I'll keep recommending (like I have a tendency to do)a couple of videos on YouTube...

for sealing first aid supplies... USNERDOC
for long term food storage... Wendy DeWitt

I got mine to start with some basic long term food storage... put away some basic bulk stores in vacuum bags in 5gal buckets...I've changed after watching the DeWitt video to utilizing the Ball/Mason jar adapter, and dry pack canning in glass...

I've put away some M855 ammunition, but don't see any advantage over the 50cal ammo cans... it's much more difficult to access, and I'll probably load it into bandoleers and repack it to a 50cal can...it might be a good idea to store .22lr in vacuum packs as the bullet is not sealed like most centerfire cartridges...I'd probably use glass for that too...YMMV

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?

if you haven't already used the vacuum sealer, it will pull the plastic down pretty tight, and there won't be much volume of air available...you might do a practice run and seal a small amount of "damp rid" and see if any visible moisture is condensed...if you seal in an air conditioned room, I don't think you will need it

I seal lots of stuff; matches and fire gear, food, batteries, dry socks, etc.

There are all kinds of videos on Youtube and other places showing people vacuum sealing all kinds of things. I have even seen them vacuum sealing firearms.

I actually think for longer term storage of rarely used firearms that vacuum sealing may well make a lot of sense, despite how it may sound at first glance.

I know some coin collectors seal up coins to keep them from tarnishing. Even slabbed coins are not air tight and can tarnish over long periods of time.