There is considerable controversy on how to define true vitamin D deficiency. The following is a general consensus (opinion of experts).
Serum total 25(OH)D
<10 ng/mL = Vitamin D deficiency
10-29 ng/mL = Vitamin D insufficiency
>30 ng/mL = optimal Vitamin D
From the laboratorian point of view, it is VERY important to note that these are NOT reference ranges based on a population of samples because many ambulatory, asymptomatic, ostensibly healthy people have vitamin D insufficiency. Much like a fasting cholesterol level, these are clinically recommended ranges.
Most agree with the definition of deficiency. However, the definition of insufficiency is controversial as demonstrated in the next several slides.