It's easy, just follow the few step below.
Step 1: To the official redemption site, click on the first picture "Mua"
Step 2: Fill in your player ID and select Zing Card as a payment method.
Step 3. Select the equivalent game card you have purchased. [Eg: 20,000VND~1,000,000 VND]
Step 4. Enter Card No. and Pin to complete your recharge.
You have done the recharge, relog your game to refresh the in-game UC.