Use the Mobifone mobile application, select the section Top Up with a scratch card and enter the code which sent on your email.
⏩ How to check my balance on Mobifone in Vietnam?
To check account balance use *101# and finish with the dial button.
⏩ How can I extend my sim card Mobifone validity?
You may also keep your number for up to 180 days without using service. Just send an SMS to 999 with the word GS30 from your Mobifone. Note: Keep in mind that there will be a fixed fee which will be deducted from your balance. There are also 360 day extensions: send an SMS to 999 with the word GS50.