You start with Vit C. Always. It needs to be on clean skin to absorb properly. You can apply BHA right after, but if the pH of the BHA is higher, it might negate a little of the effects of the Vit C.
The general rule is: acids first, then other actives, then moisturizer (if needed).
Applying moisturizer immediately afterwards doesn't necessarily "seal" anything in...when it comes to acids, it can negate everything because it neutralizes the acid before it has a chance to work. To get maximum effect, wait 30 minutes after applying acids before applying anything else. The skin neutralizes acids on it's on in about 30 minutes.
Never use Vit C and copper peptides at the same time of day...spread them out so that you use Vit C in the morning, and CP at night. Vit C and copper interact and neutralize each other. They also turn you orange...basically it's "rust". And it doesn't smell very good either...very metallic.