Not at all
Most adults need 5-8000 iu of D3 a day during fall and winter months to get adequate D.
It is possible to get D from sun in spring & summer if you get enough sun. D is best absorbed with or after a meal.
Vit C can be taken any time and it is totally safe to take C, D and all other vitamins together. If this was not safe then multivitamins would not be popular and beneficial!