Vitamin C and Copper: a layering primer
They do not play together. Here is the routine that lets you have both without one canceling the other.
Ascorbic acid (vitamin C) chelates copper. So does GHK-Cu. If you apply them together — vitamin C in the morning, GHK-Cu serum on top — the vitamin C will pull the copper out of the peptide complex and you will have neither.
The simple fix
Time-separate. Vitamin C in the morning under SPF (it is photoprotective). GHK-Cu at night, on bare clean skin, before any retinoid. You get both, fully active, without the chelation problem.
The other thing nobody mentions
If you're using a vitamin C serum and a copper-peptide product in the same routine and your formulator hasn't addressed the chelation problem, you're paying for two ingredients but functionally using one. Ask. If the answer is some version of "they're both antioxidants so it's fine," that is not the answer to the question you asked.