Selena:
I think that there is a list somewhere on applesnail.net that lists the calcium content of all types of foods you could add to your snail food. I would use the amount of vitamins that the people on there use to be safe. I wouldn't use human vitamins, I would imagine they would be way to concentrated for snails. I don't know if the vitamins are really necessary as long as you use a variety of things in your snail food.
Some recipes also use dog biscuits, I believe...
I use Maria's snail biscuits. Not sure if her site is back up yet or not. My snails really seem to like them. I have two kinds--fruit and vegetable. They seem to like the veggie ones best.
Best of luck with your project!
Laura