There is absolute mountains of literature on vit D deficiency being concurrent with many different chronic diseases, but very limited data that replacing it actually improves outcomes outside of very few specific circumstances.There is a pretty strong association between vitamin D and the severity of sleep apnea. It doesn't mean there's a causal relationship, but it's worth trying to address it for the direct and indirect benefits of being vitamin D sufficent while the slow research process plods along depending on funding.
So start popping the D pills or eating the wild salmon, UV treated mushrooms, etc.
Vit D deficiency is probably a sign of being "sick" rather than causing sickness.