My bank are already on it, unethical but I’ll simply say I never received the vouchers
See what they say first, but I reckon, you better say that you were never purchasing any gift vouchers at all!
The most important thing here is your ability to prove that the service is not as promised.
Here are a few events to consider:
1. a personal account is blocked; at the time of the transaction, a technical malfunction occurred and the money disappeared from the account;
2. at the time of making a deposit, a bonus was added to you, and now you are "forced" to make another deposit in order to meet a wagering requirement
3. withdrawal requests are repeatedly rejected; the site is closed for technical reasons / it is impossible to open the site or access your personal account;
4. you are accused of fraud, money laundering - in general, under any excuse, they block the withdrawal of funds, exposing you to blame for the situation.
5. Fake games (this one is easy provable)
I guess you are not the 1st and not the last that got scammed by these people. Any bank has a list of suspicious sites that have generated tons of complaints from their consumers, and believe me, they know that list very well!
So If your bank is already familiar with these... your deposit can get returned very soon. But keep in mind, that in different countries they treat the chargeback differently too; US, UK, Sweden, Germany etc.