Well, the exception being reptile which frequently eat prey items with BONES in them, such as mice or fish. I don't think you need as MUCH calcium in this instance, as you won't OD on it, but much of it will be excreted n their stool. Granted the extra vitamins will help, but you also need to consider a few things for this:
A.) Do you have a UVB on your Snapper?
1.) Snapping turtles are largely nocturnal so there's a good chance dusting with Vitamin D may not have benefits if they don't have UVB or sunlight to process it. The jury is still out on that one unfortunately and studies are still being done to see how this works in nocturnal animals. If you have UVB, I would go ahead and dust w/ D3, if not, just use the dust without D3.
2.) Some reptiles which were once largely thought to be nocturnal are actually crepuscular (hunts from dusk to dawn), therefore in the wild they DO see sunlight in low amounts, therefore will process modest amounts of D3, so over the past few years keepers of Leos have found D3 to be beneficial in such animals. However if a UVB is not being used, might be better again, not to use D3.
3.) Observe the stool and keep a watch for white in it. Unlike fish where white is bad, reptiles excrete excess calcium in their stool regularly. If you notice obscure amounts of white in the stool, cut back dusting or cut back on the amount of vertebrates you feed them.....what you are seeing is simply calcium that hasn't been processed because they have already absorbed the needed amount.
Ultimately, it's up to you to research each particular reptile to find out how these factors influence and affect them.
