Yes, you can use vitamin c serums and collagen serums together because they work slightly differently. Vitamin c serums are great for brightening your skin and making it look more healthy and youthful.
Collagen serums are great for plumping your skin and making it look more youthful. It will depend on the concentration of vitamin C in the product that you’re using. If it’s a high enough percentage of vitamin C, then it can cause the collagen to break down over time, which means that you’ll end up with less collagen and more vitamin C in your skin.
The longer answer is that vitamin C and collagen are both great ingredients that can help you improve your skin. Vitamin C is an antioxidant that can help you fight free radicals and collagen is a great ingredient that can help you firm and plump your skin.