Will using a vitamin C serum after microneedling impede healing? Could it ruin the treatment?

Vitamin c serums help in maturation process of the collagen and should actually help after microneedling. 

No, you didn't ruin your result, but you may have some redness and/or inflammation. I recommend following up with a cosmetic dermatologist for skincare and after care recommendations. We suggest all patients follow up with facials like Silk Peel Dermalinfusuion to hydrate, exfoliate and provide the skin with antioxidants. Sun protection is also essential. I always combine microneedling with prp and lasers to get the best outcomes. We do treatments every 3-6 weeks. Best, Dr. Emer

Vitamin C serum after microneedling helps your skin to recover from the treatment and increases your collagen production. Unfortunately not all vitamin C serums are created equal. I recommend my own serum because it is not acidic and will not irritate your skin. Also you need a product high in hyaluronic acid for maximum hydration. 
Love your skin.