Both are very important for your skin. I would recommend you use the Vit A at night and Vit C in the morning.
Both vitamin A and vitamin C have numerous beneficial effects for the skin and they can definitely be used together. Vitamin A (topical retinoids and dietary supplementation) improves the quality and texture of skin and is commonly used for its anti-aging effects. Vitamin A helps to minimize fine wrinkles, improves elasticity of skin, increase collagen synthesis, treats sun damage and gives the skin a more youthful appearance. Vitamin A can also be used to treat certain skin conditions such as Acne, Rosacea, Keratosis and more.
Vitamin C is an excellent antioxidant which protects the skin from the damaging effects of free radicals, which promotes aging of the skin. Free radicals are caused by stress, sun exposure, pollution, toxins, poor diet and habits such as smoking. Vitamin C also promotes collagen production which leads to firmer skin and regeneration of damaged skin. It also lifts tiredness from the skin, leaving the skin looking fresher, brighter and rejuvenated. Serums or creams containing vitamin C, as well as dietary supplementation with vitamin C will have positive effects on the skin and will protect the skin from damage and aging.