Not to get too off topic, but the reason we've lost the ability to synthesize our own vitamin C is likely because of the existence of those fruits in our diets that provided it.
Most animals synthesize their own vitamin C, and if a mutation came about that compromised that, the animal would likely not survive, reproduce, and spread that mutation. But for humans, other apes, certain bats/bird species etc, (which already ingested vitamin C foods in their diet) that mutation was irrelevant and did not affect survival enough to be weeded out.
I feel like if anything, this whole Vitamin C flaw that we have is a perfect example against intelligent design.