Here are the steps to redeem the voucher.
1) Make a reservation on www.vietjetair.com in Japanese Yen (voucher covers up to JPY45,870) The ticket class must be Eco or higher.
2) Choose "PAY LATER" as the payment method.
3) Send me the flight details and I will direct VietJet to use my credit on your ticket.
Unfortunately, VietJet does not have an online tool for redeeming credit vouchers, so I will need to communicate the new flight details to them directly via email.