Hello~It is very unlikely that it is due to the Vitamin C, I have taken that brand in the Raspberry flavor, it is red, and it has never changed the color of my urine. You actually can not get too much vitamin C, it is a water soluble vitamin, thus your body excretes what it doesn't need, but again, it would not be dark unless you are not drinking enough water throughout the day.
My first thought was that you are not drinking enough water, if you body does not get enough water, your urine will be a darker yellow. How much water are you drinking? You should drink at least 8 8 ounces a day, preferably more if possible.