I agree that it is possible to overdose on vitamins... I HAVE! Just try taking 2 prenatal vitamins a couple hours apart and see how you feel! Besides D3 supplementation shouldn't be necessary in nocturnal animals. My leos are nocturnal too and I have never supplemented their D3 though they have calcium dishes.
I think the best way to make sure your snake is getting what he needs is to make sure that the prey you are supplying to him appears healthy and was well cared for. That's why I breed my own prey.
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All my Children: 0.2 female humans (1 is 10 and the other is a 6 month old hatchling), 1 Saint Bernard, 2 maine coons, 15 leopard geckos, 1 pacman frog, 1 oscar, 1 jack dempsey, 1.1 ball pythons, and "Lilith" the black meowing kitty