You would have to take 50,000 international units for 6 months for it to reach toxic levels. Even then all you do is stop taking it and you are then fine. 5,000 iu is now a recommended dose for most people defiecient.
Vit c is a different story. There is no stock standard level of overdose, It will depend on what other medications are being taken and if they interact.
Continue with your Vit D especially. This will be monitored and if they doc thinks you need to lower it, they will tell you, but rest assured you won't become toxic on the dose you take.
Cheers
My doctor prescribed 50,000 IU/wk; it's not an overdose if you're defficient. I've now REDUCED my dose to 5K IU/day of Vit D.