There was a study I saw that showed much more absorption into the blood when people took vitamin D with a meal that has fat or oil in it, rather than taking the vitamin D with just a carb snack with no fat or oil. The worst absorption was taking it on an empty stomach. I didn't keep the link to that study.
This is an easy thing to do, taking the vitamin d with a fatty meal, and it might bring up blood levels in people.
On another angle of all this, a book I recently read (name of book is below, but the book itself isn't really relevant to this thread), the author cited bits of evidence that the best time of day to take vitamin D is early in the day, and that if we wait until dinner then it does not do as much good. I found that informatio to be odd, because vitamin D is stored well, supposedly, in the body. Taking vitamin D early in the day is an easy thing to do, so I am now doing this, it cannot hurt to change the time of day you take it, and according to that book it may help.
I don't have feedback on health changes linked to blood levels of D though, I have not tested mine in quite awhile.
Interesting idea though!
The book was called "The Perfect Health Diet", if anyone wants to read that part about the vitamin D the author wrote. Book is on Amazon. The rest of that book is not about vitamin D though.