If you are trying to use it to treat mild depression specifically, a placebo effect is great! Placebo effects get a bad rap. If something improves your subjective quality of life that should be considered a win, especially for depression.

Honestly the best thing vitamin D has going for it is being super inexpensive and safe. Because of that you can supplement vitamin D for almost any reason and expect to see at worst a very small cost (someone up thread said 3 cents a day, which seems in the ball park) and at best a significant subjective benefit (even if it's from a placebo effect).