You could look into an app like ViralSweep which gives users incentives to complete an action such as giveaways. I'd contact them to see if their app could help you set up an action for a giveaway when the user adds their review.
This app, called Judge.me, allows you to give coupons when a review is placed. You may be able to give a coupon for 100% off your Vitamin C Serum, I think that might be a great solution.
Hope this helps!
I did try to do some research but didn't find either.
Why don't you try to send free products to Influencers/Youtubers in exchange of a product review/placement?
There are still many influencers willing to collaborate on a barter basis, you will get exposure for your brand as well as the reviews that you are looking for.