There is emerging evidence to suggest that vitamin D deficiency is associated with adverse outcomes in COVID-19 patients. Conversely, vitamin D supplementation protects against an initial alveolar diffuse damage of COVID-19 becoming progressively worse. The mechanisms by which vitamin D deficiency exacerbates COVID-19 pneumonia remain poorly understood. In this review we describe the rationale of the putative role of endothelial dysfunction in this event. Herein, we will briefly review (1) anti-inflammatory and anti-thrombotic effects of vitamin D, (2) vitamin D receptor and vitamin D receptor ligand, (3) protective role of vitamin D against endothelial dysfunction, (4) risk of vitamin D deficiency, (5) vitamin D deficiency in association with endothelial dysfunction, (6) the characteristics of vitamin D relevant to COVID-19, (7) the role of vitamin D on innate and adaptive response, (8) biomarkers of endothelial cell activation contributing to cytokine storm, and (9) the bidirectional relationship between inflammation and homeostasis. Finally, we hypothesize that endothelial dysfunction relevant to vitamin D deficiency results from decreased binding of the vitamin D receptor with its ligand on the vascular endothelium and that it may be immune-mediated via increased interferon 1 \alpha. A possible sequence of events may be described as (1) angiotensin II converting enzyme-related initial endothelial injury followed by vitamin D receptor-related endothelial dysfunction, (2) endothelial lesions deteriorating to endothelialitis, coagulopathy and thrombosis, and (3) vascular damage exacerbating pulmonary pathology and making patients with vitamin D deficiency vulnerable to death.