He needs the d3 to help process the plain calcium properly. The plain calcium can't do everything on its own, neither can the d3. That coupled with the proper UVB exposure is what 'prevents' mbd. You use the plain calcium most feedings, the d3 twice a month and of course have a UVB light always unless they get a lot of outdoor time.
The balance between the various supplements is very important, too much or too little can cause problems.
Well just make sure you use a great gutload in the meantime then he needs those vits too, to help with his aim and sticky tongue, his skin and shedding, eyesight and so on
It's only about ten bucks so I'm sure you'll manage