There are many methods for preserving vitamin rich fruits and vegetables. Jams, preserves, dried fruits and pickled fruits/vegetables will all retain some if not all of the vitamins they would have if eaten fresh. High sugar, salt or vinegar content can preserve vegetables and fruit for months or even years. These techniques have been in use since ancient times and were certainly employed in Europe for many centuries before potatoes became available.

I hope this source is acceptable? I haven't attempted to answer a question on this sub before http://nchfp.uga.edu/publications/nchfp/factsheets/food_pres_hist.html