Sorry, you've made things look quite confused.
Did you mean in 1mL of orange juice there's 10mg of vitamin C? If that's the case, and you want to find the mass of vitamin C needed to decolourise 1mL of 1% DCPIP, then times 10mg by the volume of juice used.
For example, if it takes 5 mL of juice to decolourise the DCPIP, and there's 10mg of vit C per 1mL of juice, then it's 50mg of vit C.