K/DOQI recommends spending 3-6 months correcting 25-OH vitamin D deficiency prior to graduating to active vitamin D to control secondary hyperparathyroidism in CKD patients. I have been aggressively treating vitamin D deficiency in my CKD clinic for years and have found a pretty modest affect on PTH. Generally you get 20-30% reduction in PTH by correcting 25-OH D but occasionally you get a real responder.
I just saw a patient with modest stage 3 CKD. When I first saw her she had a 25-OH D of 7 and a PTH of 288. I started her on 50,000 units of ergocalciferol a week. It took 12 months but we finally corrected her vitamin D deficiency and her secondary hyperparathyroidism just melted away.