The regulations concerning vouchers has become a national law since 1/1/2019. This only concerns vouchers that give notification of conditions and vouchers that can be exchanged for a delivery of services or good, thus it does not include discount coupons.
For so-called single vouchers, or vouchers where the VAT is already known, the VAT is payable from the moment the voucher is made up. For instance a voucher that you can redeem for a beverage basket. Hereby, you already known the VAT (21%) beforehand. In case the voucher is not redeemed within the agreed period, the VAT cannot be recovered.
In case of multiple use, or vouchers where the VAT amount is not known beforehand, the VAT is payable when the voucher is redeemed. For instance, a voucher for a restaurant visit. When you make up the voucher you don’t know whether the person will use it for beverages (21%) or for meals (12%). In case of non-use of the multiple vouchers, there will be no VAT since the VAT is only calculated when redeemed.