This is one of those problems in medicine that just does not go away. Each time you think the issue is settled, researchers who believe in the magical powers of vitamin D continue to come up with studies trying to show its usefulness in everything from reduced mortality to reduced risk of all kinds of diseases. The amount of energy and money that vitamin D research sucks away from other important research questions is far out of proportion to any perceived benefits that may exist.
I have earlier written about the hype and hoopla around vitamin D and why supplementation is of no use and could perhaps even be harmful, unless you are nutritionally deprived, malnourished or have clinical symptoms and signs related to vitamin D deficiency. The earlier presumptions that vitamin D supplementation in normal individuals could increase healthspan and lifespan were mainly from observational studies…these presumptions have not panned out in recent randomized controlled trials including Vital and D-Health. The USPSTF also does not recommend routine supplementation or measurement of vitamin D levels, which unfortunately is being done more and more these days as part of annual health check-ups and in most people who don’t feel well for any reason whatsoever.
A recent study by the EPIC group  used Mendelian randomization on a large cohort of almost 500,000 plus people to show a causal relationship between vitamin D levels below 16 ng/ml (40 mmol / ml) and mortality. This potentially makes sense, because below 20 ng / ml is considered insufficiency and below 12 ng / ml, deficient and a vitamin D level less than 16 ng / ml could perhaps be associated with increased all-cause mortality.