@ysmina-- I don't think that the additional ingredients in a vitamin C serum for the face are very important. The potency of the vitamin C and the pureness of the ingredients are more important factors if you ask me.
If the percentage of vitamin C in the serum is too little, or if there are a lot of chemicals and synthetic ingredients, the serum will not be very effective. I think that a vitamin C serum should have about 20% vitamin C and the other ingredients should be all-natural and pure. You would not believe the type of chemicals that are added to some skin products.