So you think about selling yet another Vitamin D supplement. If you're famous, you just say: "hey guys, you are so great and loyal, here is my new supplement, buy it" and they do. If you're not or if you have a new website with low rank, nobody comes to your website to begin with. Given that you cannot promise anything with supplements and you can get vitamin D at every corner(at prices you cannot beat), how do you sell something like that, if you are new to the market and don't have massive funds or a huge following and are not at a competitive price.
I can't see any advantage trying to sell something like that. I think all it takes is being in the sun about 15 minutes per day to meet our daily need of vitamin D. Sunlight is free. I walk daily and garden. Grocery stores, dollar stores, drug stores all sell vitamins. Shopping for personal or family needs is time consuming. I'm convinced many of us try to to as much of one stop shopping as possible.
If new in market then need funds to make your brand popular because there are very high competition. If fund issue then i think @Smyrl is very right.