"I would make your own vit c serum. It's super easy, even the most basic mix of vit c powder & water works really well. I mix glycerine, rose water and vit c powder."
It doesn't actually. See here:
3.1.3. Topical Application of Vitamin C
When plasma levels are low, some vitamin C can be delivered to the epidermal layer by topical application, although the efficacy of this is dependent on the formulation of the cream or serum used on the skin [51,52,53,54,55]. Vitamin C, as a water-soluble and charged molecule, is repelled by the physical barrier of the terminally differentiated epidermal cells. It is only when pH levels are below 4 and vitamin C is present as ascorbic acid that some penetration occurs [56].
A great deal of effort has been put into the development of ascorbic acid derivatives for the purpose of topical application. Such derivatives need to ensure stabilization of the molecule from oxidation and also overcome the significant challenge of skin penetration. In addition, they must be converted to ascorbic acid in vivo in order to be effective. Whether there is a single solution to all these challenges is unclear [57]. The addition of a phosphate group confers greater stability and these derivatives may be converted to ascorbic acid in vivo, albeit at a slow rate [58], but they are poorly absorbed through the skin [56,59,60]. Ascorbyl glucoside also exhibits superior stability and can penetrate, but the rate of its in vivo conversion is not known [57,61,62,63]. Derivatives containing lipid-soluble moieties such as palmitate are designed to assist with delivery, and although increased uptake has been demonstrated in animals [64], they do not necessarily show improved stability and there is some doubt as to whether these derivatives are efficiently converted in vivo [57]. Recent studies suggest that encapsulation into a lipospheric form may assist with transport into the lower layers of the epidermis and could result in increased uptake [65,66,67].