It would likely be unethical to conduct a study to determine whether CVD risk is affected by vitamin D supplementation in those with vitamin D deficiency, becau

It would likely be unethical to conduct a study to determine whether CVD risk is affected by vitamin D supplementation in those with vitamin D deficiency, because if you determine that study participants are vitamin D deficient then it is indicated to treat them regardless of impact on CVD risk. Thus the trial does not attempt to do such a thing, focusing only on a general sample, excluding anyone in whom vitamin D supplementation would be contraindicated. Ultimately while it would be interesting to know if CVD risk is reduced with vitamin D supplementation in those who are vitamin D deficient, it is largely an academic question as those individuals should be taking vitamin D supplements anyway.