Short answer: niacinamide, yes—vitamin C, it's complicated. Since niacinamide is barrier-supportive and low-irritation, it layers well with retinal in the same routine, even the same night, without conflict. In fact, pairing the two can work in your favor: niacinamide helps calm any dryness or sensitivity that retinal might cause, making it a smart addition rather than something to avoid.
Vitamin C is a different story, mainly because of pH. Vitamin C (especially in its most effective form, L-ascorbic acid) needs a fairly low, acidic pH to stay stable and active, while retinal performs best at a more neutral pH—so applying them at the same time can reduce the effectiveness of both, and in some cases increase irritation. The more common approach is to separate them by time of day: vitamin C in the morning to help with environmental protection and uneven skin tone, and retinal at night as part of your night time skincare routine, when it won't compete with vitamin C or get destabilized by daytime sun exposure.
If you'd rather not think about timing at all, some formulas are designed to sidestep the pH conflict altogether by using more stable vitamin C derivatives—which is worth checking for if you want to simplify your routine without giving up either ingredient.
That's exactly the approach behind KSECRET SEOUL 1988 Niacinamide 5% Capsule Cream: instead of pure vitamin C, which is sensitive to light, heat, and air and can easily irritate skin, it uses a more stable vitamin C derivative that's gentler and safe to use alongside retinal. Its moisture gel and vita capsule formula also help soften the dryness, tightness, and flaking that can come with retinal use, forming a protective moisture barrier as it works. Pair it with the Retinal Liposome 2% Serum at night for a routine that covers both bases—retinal's potency and vitamin C support—without the usual layering conflict.