See the point is we generally provide a co-factor or a immuno booster in situations where people usually are deficient in certain vitamins and minerals, like pregnancy/elderlies/children or who otherwise have other medical health problems and cannot take adequately balanced diet.
Vitamin C is for vessel wall stability and strengthens the epithelial barrier which adds to number 1 immunity.
Vitamin D is a well known immuno-booster. Its role has been well studiwd even in diabetics. You can take Vitamin C 250 yo 500 mg dailyaccording to the kind of diet uou are consuming. Vitamin D can be safely taken upto 60k units per week. It is a fat soluble vitamin, so overdosing may have subtle side effects.