Thanks for providing a plausible explanation. Do you know of any links (ideally peer- reviewed research) supporting the quick evolution of resistance to vitamin C? If not I'll google around.

That's a generic cancer resistance mechanism. That's why most of cancer treatments fail eventually, the cancer cells evolve to tolerate the levels of chemotherapy agents that are lethal to regular cells.

In the articles linked in this thread, large-dose vitamin C prolonged survival, but didn't clear the cancer. It's exactly what you'd expect from a chemo treatment.