I've read a lot about layering skincare this week and since around February (it's May now) I've been using tretinoin almost immediately after my Vitamin C serum. I haven't had irritation but I'm wondering if this combo caused some permanent damage in my skin? I read that it is advised to wait 30-40 minutes after applying Vitamin C. Also, would it be advisable to use AHA/BHA's in the morning, and tretinoin at night? Or AHA/BHA's twice a week and tretinoin for the days not using the acids?