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Vitamin D below 20 ng/mL is definitely correlated with bad things. Anecdotally, guys who were deficient by this standard have achieved better testosterone levels via supplementation. Overall though vitamin D supplementation is striking out big-time, with the new, large studies not finding improvements in all-cause mortality and other parameters. It's been suggested that low vitamin D is serving as a marker for underlying health issues, which supplementation does not address.

I pick 30s ng/mL because it's nicely between the extremes where there are correlations with bad things (<20, >50 ng/mL). Note that 50-60 nmol/liter is 20-25 ng/mL. I don't recommend that low because other studies may suggest higher. But I also don't think people at that level should panic. The decision by the labs to raise the lower limit to 30 ng/mL is not well-grounded.