Moral of the story: be the kind of person who takes vitamins, and then don't worry about actually taking them.
In addition, there are tons of substances that your body can synthesize itself, but would love more of it. Creatine for example can be made in the liver and kidney from L-arginine, glycine, and L-methioninein, but it takes work for your body to do it. It is much more efficient (something like 20X) to just take a creatine supplement and bring it to levels that will allow your body to maximize it's production of the energy molecule ATP (by resupplying a phosphate molecule).
But we know your first claim is not true, because the body can make Vitamin D by itself. Just because it was called "vitamin" isn't enough to mean anything.
And the second claim is just what the article is disproving!
And then more recently there is evidence that taking vitamin supplements leads you to take risks with your health: http://www.theguardian.com/commentisfree/2011/aug/26/bad-sci...