One medium-sized baked (or roasted) potato with the skin has 17 mg vitamin C, so you'd need to eat about five baked potatoes (87 mg vitamin C) to get a day's worth of vitamin C.
Those five potatoes (served plain) would also have 649 calories and contain 10 mg niacin.
As much as I love baked potatoes (and I really love baked potatoes), I don't think I'd want to eat five of them just to get my vitamin C. Still, a potato a day isn't a bad idea:
United States Department of Agriculture National Nutrient Database for Standard Reference. "Potatoes, baked, flesh and skin, with salt." Accessed October 22, 2010. http://www.nal.usda.gov/fnic/foodcomp/search//.