I said I wouldn't vac-pack a loaded weapon--your luck the vacuum would pull the trigger! lol But I would vac-pack the ammo to go with it any day!A few years ago I seen bags made fer that, beleive they was impregnated with cosmoline. Haven't seen any in a spell though.
I'd think if ya put a rag with a bit a gun oil in there, clean em well they should be just fine.
Ammunition is vac bagged all the time.