VAT on voucher sale?

I am helping to set up a group buying site (VAT registered), but am having difficulty figuring out the VAT issue. With regard to group buying sites such as groupon, livingsocial etc, who actually pays for the VAT?

As a customer, when one buys a discounted voucher for £20 on something of a £100 value, one pays £20 and when redeeming, does not pay any more.

Therefore, if the merchant agreed to sell the product / service at a discounted rate of £20, and £10 goes to the group selling site, does the merchant pay the VAT on the £20 or on the amount actually earned; £10, or is the VAT owed to HMRC split between the merchant and the group selling site?

Or, is the voucher considered a gift certificate without VAT? Who is responsible for the VAT?

Hopefully someone knows the answer to this as seems to be a very complicated issue.

Many thanks in advance.