Vitamin C for skin lightening. A day?

yes. you can take it. they are safe. but that alone might not be enough. The darkening of skin can be multiple causes like tanning, vitamin B12 deficiency, addisonian pigmentation, acanthosis nigricans, lichen planus pigmentosus, rhiel's melasnosis or several other causes. A proper assessment and examination will help ascertain the proper cause. For loss of glow & dullness - there are antioxidant therapies like glutathione , lasers for rejuvenation, PRP therpy for facial rejuvenation , peels, etc. Kojic cream alone is safe but less effective. You can try using this for a month twice daily on the dark spot. The BEST cream would depend upon the cause. Get an online consult privately with pictures or meet a dermatologist directly for a proper evaluation of the condition. Hope this was helpful.