Vitamin C is heat sensitive but fermentation is fine and a good reason why fermented cabbage is popular in places with cold winter. See kimchi and sauerkraut, as rice or rye alone would kill you over a long winter. Similar mechanics going on for andean freeze dried potatoes to a lesser extent. Beyond that, it's straight up foraging for greens and berries but that only really works if you're moving a small enough group of people to allow forage to be an option. Plenty of leafy greens from forage allowed enough vitamin c to stave off scurvy for many ancient armies and sailors(though not all). Cook notably would beat sailors who wouldn't eat foraged greens. The other option was uncooked organ meats.