Nope, the guns will work fine in vacuum, at least as far as firing goes. The shells contain their own oxidizer. The only worries are potential vacuum freezing of moving parts, contamination by dust electrostatically attracted into the weapon, or one of hard freezing causing the metal to become brittle. And it's not clear those are likely concerns or not. But they will fire fine in vacuum, It's an old Traveller debate
Same for the flares I think, an oxidizer is included in the chemical in at least some I've had experience with. Maybe not magnesium flares though, and I'm not too sure what was in the ones I had at the time or if my attempt to extinguish by cutting off air was sufficient to prove they had their own oxidizer