If your hypothesis is that timing matters but the blood level doesn't matter, what's the underlying biochemical mechanism? Is there any evidence that the right time in the circadian cycle is crucial?

I see evidence that both matter to sleep. It seems that the blood level of vitamin D is linked to excessive daytime sleepiness. (Warning: the authors of this paper overfit the data, so you can ignore their conclusions about race and very low vit. D levels, but their data does show a negative correlation between serum vitamin D levels and excessive daytime sleepiness.)

My own serum vitamin D level was pretty low, but since supplementation, it has increased appreciably to within the normal range. So far, I don't think it has had an effect on my daytime sleepiness, but I have not been keeping track of the appropriate factors, so take what you will.

It also seems that taking vitamin D at night seems to disrupt sleep for some individuals. My experience suggests taking vitamin D at night has no effect, but (as before) I have no hard data to justify this. It is possible that the sleep disruption only applies to those who have adequate blood levels of vitamin D. The explanation that I have seen (which I can't find right now) is that vitamin D influences your circadian drive as sunlight would because your body synthesizes it from sunlight; taking vitamin D is like getting "concentrated sunlight". I'll agree with ChristianKI, though, that no mechanism needs to be identified to validate an observation.

For other things (i.e., not sleep), I haven't seen any evidence of timing effects.