No, they do not automatically “cancel each other out”
The idea that vitamin C and retinol can never be used in the same routine is an oversimplification. Modern products are formulated systems, not beakers of isolated ingredients. The practical issue is tolerance: an acidic L-ascorbic-acid serum can sting, and retinol can also irritate, so using both at once may simply be too much for some skin.
The easiest routine is usually vitamin C in the morning
A morning antioxidant serum followed by moisturizer and broad-spectrum sunscreen, with retinol reserved for the evening, is popular because it separates two treatment steps. It also makes it easier to identify which product caused stinging or dryness. The split is a convenience strategy—not evidence that the ingredients are chemically incompatible.
Same-night use can be reasonable
If both products are already comfortable on their own and their directions do not conflict, some people can use them in the same evening routine. Introduce that change deliberately: do not simultaneously increase retinol frequency, switch cleansers and add an exfoliating toner.
“Vitamin C” is not one formula
L-ascorbic acid, sodium ascorbyl phosphate, tetrahexyldecyl ascorbate and other derivatives can feel very different on skin. Concentration, pH, solvents and the rest of the formula affect tolerability. That is why blanket internet rules about “vitamin C + retinol” often fail to describe the products actually in your bathroom.
When separation is the better choice
If either product stings alone, keep them apart. If the combined routine causes persistent redness, tightness or peeling, alternate times of day or use vitamin C on non-retinol days. Consistency with a tolerable routine is more useful than proving that every active can coexist.