Where should I incorporate a vitamin C serum into my regimen? I use Aczone in the morning and Tretinoin at night.

You would want to use a Vitamin C in the morning underneath the Aczone. Make sure you are using an L-ascorbic acid form of Vitamin C. iS Clinical Pro-heal is a great choice. After you put it on, let it sit on the skin for about 5 min before applying the Aczone.