I don't know how it was approached for vitamin D, but it's all about the model they choose, which in the first instance is just something they pull out of thin air. For many water soluble vitamins and minerals the model is based on a threshold for urine excretion; up the dose until the study group is excreting as much as they take in. Until someone figures out otherwise--i.e. that it's too little, too much, or that other considerations need to be made--that's the basis for the RDA.