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.

Here's a really interesting article on how it was discovered that citrus would help. They were also able to preserve citrus and citrus juice with alcohol. They could also turn it into a concentrated syrup without too much loss of vitamin C.

From what I just read, they didn't do this, but dried citrus, when dried at a cool temperature, retains the majority of its vitamin C.

Cabbage.

Jam is the classic way to preserve fruit.

https://www.usni.org/magazines/naval-history-magazine/2021/february/finding-cure-scurvy

Gilbert Blane was appointed to the staff of Admiral George Brydges Rodney as Physician to the Fleet in 1779. Blane was a medical reformer who was convinced by Lind’s original experiment with citrus and appreciated the need for a practical way of storing them. After considerable experimentation, he determined that adding 10 percent “spirits of wine” (i.e., distilled ethyl alcohol) to lemon juice would preserve it almost indefinitely, without destroying its beneficial properties.

Fermented cabbage, AKA Sauerkraut.

• Smoked Meats
• Dried Meats
• Pemmican
• Jerky

To prevent scurvy you don't need megadoses of vitamin c - you need tiny amounts of real meat with no glucose. Glucose and Vitamin C both compete on the glut-4 transporters - so in a modern high glucose (carbohydrate) diet, you need a bunch of vitamin C to win those competitions. In ancient diets glucose load wasn't really a factor, so the meat is sufficient by itself.

The most common sufferers from sucurvy were sailors eating hard tack

and not eating much else. That is basically a fully carbohydrate diet, which means lots of glucose, which means lots of glut-4 competition for the little meat they did have in their rations.

Pemmican appears to be the ultimate survival food, fueling ancient expeditions across the americas and the arctic. It's a mix of dried meat and suet (fat), very energy dense, provides complete nutrition, and extremely storable for years/decades as long as it is kept dry.

Citrus.

British sailors got the moniker "limey" because they usually had limes specicially to ward off scurvy.

It's not ancient but blackcurrant syrup aka ribena was originally developed for this purpose when other fruit supplies were running low in Britain during the war

In fruit.

Got a spruce tree? Grab the newer green tips on the branches and check it in some hot water. That tea is full of Vit C, and tastes like Christmas smells. And the tree isn't going to go bad.

Dried chili peppers are a good source

Maybe something that could be dried, soybeans could work. Would have to be properly stored to keep it dry.

Yeah. Fruit! Lots of fruits have vitamin c.

Potatoes aren't too bad a source IIRC.

I'm kind of surprised no one has said honey. While I can't find exactly when honey was first used as a preservative, it's believed that it was first used in ancient Egypt. I also don't know if vitamin C specifically was ever preserved in honey but honey has been used as a preservative for fruit.

oranges, anything related to citrus family.

In a living body impaled on a spike in Vlads front yard.