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.

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.

Cabbage.

Fermented cabbage, AKA Sauerkraut.

Jam is the classic way to preserve fruit.

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

Dried chili peppers are a good source

In fruit.

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.

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