Vitamins that are reducing agents can be pro-oxidants.  Vitamin C has antioxidant activity when it reduces oxidizing substances such as hydrogen peroxide, however, it can also reduce metal ions which leads to the generation of free radicals through the Fenton reaction.
::2 Fe + 2 HO → 2 Fe + 2 OH· + 2 OH
:2 Fe  +  Ascorbate  →  2 Fe  +  Dehydroascorbate
The metal ion in this reaction can be reduced, oxidized, and then re-reduced, in a process called redox cycling that can generate reactive oxygen species.
The relative importance of the antioxidant and pro-oxidant activities of antioxidant vitamins is an area of current research, but vitamin C, for example, appears to have a mostly antioxidant action in the body. However, less data is available for other dietary antioxidants, such as polyphenol antioxidants, zinc, and vitamin E.