I suspect that the concern with Vitamin C is a red herring. Given that β-hydroxybutyrate activates the body’s defences against oxidation, thus greatly reducing or possibly eliminating the need for exogenous anti-oxidants, the question of what the benefit of Vitamin C is on a ketogenic diet is far from clear.
What we do know, and have known for at least four centuries, is that people who eat fresh meat do not get scurvy. But what the property of fresh meat is that prevents scurvy is not, to my knowledge, known. We don’t even know what the Vitamin C content of meat is. The U.S. Department of Agriculture says “assumed to be zero,” which means that no one has even done an analysis. And it could even be that fresh meat “prevents” scurvy simply by occupying a large enough percentage of the diet to keep insulin low enough to prevent it from shutting off our exogenous defences against oxidation.
By the way, the British sailors who developed scurvy on long deployments away from harbour did not develop the condition until the livestock aboard ship had all run out, and their diet switched to ship’s biscuit (hardtack) and salt beef. Since they were not eating their meat raw (I assure you), that means that if the anti-scorbutic property of fresh meat derives from its Vitamin C content, then Vitamin C must survive cooking in sufficient quantities to be therapeutic.