If you read through the many studies done, Vit C does not cause kidney problems. It has tremendous health benefits and it's impossible to overdose on it as the body will just eliminate what it doesn't need.
This is observable even in myself. I can take about 20 grams per day as a maintenance dose of Vit C. If I exceed that dose and go up to 25 grams, I get the runs - it never fails. However, if I get a cold (it's been forever since I've had one now) I could take up to 60 grams per day without an effect on my bowel movements. The plus was that all that Vit C just kicked the snot out of that cold - literally. It was gone in about 24 hours.