It takes a LOT of effort. Like in excess of 50K UI a day for months. And then it needs co-factors to do harm, like excess calcium intake along with a K2-MK7 deficiency and/or a potassium deficiency (both of which are, unfortunately, very common). All in all, it's a very complex system that interacts with the body's buffering mechanisms.

From Wikipedia: “according to some research, the tolerable upper intake level (UL) is 4,000 IU/day for ages 9–71 (100 µg/day). Other research concludes that, in healthy adults, sustained intake of more than 1250 μg/day (50,000 IU) can produce overt toxicity after several months”

Of course literally everything is harmful in excess, including essentials such as water and oxygen. The harmful dose being very different to the normal/helpful dose is as good as it gets.