I don't think "compared to not taking any at all" is an accurate summary here. Even if you never touch a Vitamin D supplement, many people are already getting an amount comparable to the baseline used in this study from a typical diet, or from sun exposure.
2/3 of Americans get the RDA of Vitamin D. Picking a specific outlier case for your control group wouldn't make a lot of sense for a scientific study (not to mention, as others have pointed out in this thread, would be difficult or unethical.) If you are a vegan living in a low-sunlight location, look into supplementation.
I'm saying that probably a very tiny percentage of Americans get 0 IU. Which makes 0 IU a very poor choice of a baseline. Since you need some concrete amount to use for your control group, what would you propose using?
