Data from large observational cohorts and trials designed for bone-related outcomes consistently demonstrate inverse relationships between (1) serum 25-hydroxy-cholecalciferol (25(OH)VitD) and prevalent T2DM/CVD or conversely, (2) supplementation and incident all cause mortality. Baseline serum 25(OH)VitD predicts future glycaemic status, whilst VDD associates with markers of insulin resistance, coronary calcification, aortic stiffness, left ventricular mass, endothelial dysfunction, and hypertension. Diabetes-related micro and macro-vascular complications may be exacerbated by VDD.