It is difficult to die of a Vitamin D overdose, though not impossible. Here is what happens: Vitamin D increases calcium absorption from the intestines. Over time, if you are taking too much Vitamin D, your levels can get very very high over time. Vitamin D is fat soluble, so you don't just "pee it out" like some water-soluble vitamins. It stays in you long-term. Eventually your calcium will rise. It can rise slowly, but you will get symptoms. Common symptoms would be fatigue, brain fog, insomnia, headaches, and kidney stones.
If you are at levels where you could potentially die from Vitamin D overdose, your calcium levels will be high enough that you will feel acute symptoms of markedly elevated calcium: like vomiting and confusion. Someone will end up taking you to the hospital, because you will become unconscious. At that point you will be given IV fluids and the calcium will drop. You could die, especially if you have other health issues, but more likely it would be treated and you would get released.
The effects of Vitamin D toxicity are related to the high calcium levels that it causes.
