Doing a Vitamin C color removal will only make matters worse. This sounds like your hair is severely damaged, and is in serious need of protein. Fragile, over processed, highly porous hair tends to soak up color quickly but, cannot hold the color molecules. Here are a few recommendations.
1; Invest in a sulfate free shampoo, and shampoo less often. Shampoo using lukewarm water,once a week, at the most. It should help the color fade less.
2; Use a strong protein treatment, like ApHogee 2 step, immediately before (not after) dyeing. This will help in achieving even color results.
3; When choosing a type of dye, drug store box dyes are a no-no; go to the beauty supply shop. Look for dark brown hair dyes with an "N" or neutral/natural base, and use a 10 volume developer (Wella, Clairol, Ion Color Brilliance, all have these labels) ....unless it needs a special developer ( like Clairol Colorgloss), or no developer at all.
4: Never shampoo immediately after coloring. Rinse well with lukewarm water, use a deep moisturizing conditioner, and rise with cool water.
5:Keep your hair well moisturized, minimize heat styling, sun damage. Dry hair results in faster color fade.