No, not the same effect - oral vitamin C tablets can markedly help some skin conditions, if you are deficient. Both can help skin function and health, but they work in different ways.
We humans are unable to synthesise vitamin C, so we need to get it in our diet. But even with a good dietary supply, only a small amount finds its way into the skin (although people vary markedly in exactly how much gets there). There must be a supply of vitamin C to the dermal layer to allow fibroblasts to synthesise collagen and elastin.
Most assumptions about the usefulness of vitamin C to skin are made after finding that individuals with skin problems like inflammation, poor wound-healing atopic dermatitis (eczema), psoriasis, acne and prematurely aged skin, often had low vitamin C levels in their blood plasma. By increasing vitamin C in the diet, these conditions can be improved. However, if there was already ‘saturation’ in the plasma - so enough vitamin C was getting into the system - taking more would not improve skin further.
This all goes on at the dermal layer of skin, not the epidermal, which is the bit that can be affected by applying a vitamin C serum.
Skin cells can store vitamin C, both from diet and from topical application, and when there is also a supply of vitamin E, they work synergistically to improve oxidative stress, repair UV damage, wound-healing, scar formation, reversing hyperpigmentation, collagen stability and synthesis, and some age-related deterioration. Cleverly, vitamin C can actually help these cells to store vitamin E.
The main difficulty with getting topical vitamin C to where it can be useful, is in how to get it absorbed. The best-studied version of it is L-Ascorbic acid, but this is water soluble, and skin is very effective at repelling water, so it needs to be joined to something that can get it over this hurdle. The other difficulty is that it is looking for an oxygen molecule to join to and so it oxidises very quickly when it mixes with air.
When it is mixed with vitamin E and ferulic acid, it performs better, but it needs to be in a solution with a pH under 3.8. If you can keep it sealed until it reached the skin, then it can both repair damage and protect from UV radiation. There are forms of vitamin C that are lipophilic (esters), so the water-repellent quality of skin isn’t such a problem and they can be used at the same pH as skin, around 5.5 to 6. But these may not be absorbed in useful amounts and more studies are needed. Whichever type is used, it must be applied in a concentration, in the skin (this is not always the same as what is in the bottle), of over 3% and probably 5-8% is optimal. It seems that applying over 20% does not increase its usefulness and is likely to be very irritating.
So, in short, both are useful, but ingested vitamin C is essential in preventing serious skin disease. Whereas a well formulated vitamin C serum can help protect skin and reverse some skin damage, but couldn’t really be described as essential.
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