The difference with vitamin c is, we are one of the only animals/mammals that are incapable of producing vitamin c within our own body. So we need to get it from other sources. And generally speaking, people don't come close to satisfying that with their diet. (as it would take six serves of fruit or vegetables a day to satisfy the recommended vitamin c dose per day)
Also, as a general statement rather than a reply, when taking vitamin c as a supplement, it is better to space the doses throughout the day, rather than take a large dose at once. It gives your body enough time to process each dose, and put more to use in your body, than giving it all to your system to deal with at once, where a good deal may be excreted out of you.