Do you think supplementary vitamins will be of any good to snails? Can they be given poultry multivitamins?
I don't think they need it. They can get all the vitamins they need from leaves and fruits. What they'll need is extra calcium for shell formation.
Maybe you should try it with few snails and know what happens. You may discover sth new.
I once sprinkled them with vitamin solution. They didn't die at least.