Why close your eyes to work on pronunciation?
English differs from Vietnamese in one important way: the written form tells you almost nothing about the sound. In Vietnamese, the spelling maps onto the pronunciation nearly absolutely. In English, most of the time, it does not.
The best method
When you work on pronunciation, the best approach is to close your eyes, listen, and repeat.
This is the habit called shadowing, which is simply imitation. Think of teasing a classmate at school by repeating everything they say exactly as they said it. That habit is what makes your pronunciation steadily more accurate.
As you listen, focus on the sound and then say it. Do not look at the letters. If you do, you fall straight back into the old habit of reading the spelling, and you say it wrong.
First time through, just listen. Second time, repeat.
Why this matters
Your eyes and ears are competing. When the written word is in front of you, your brain trusts the letters, because that is what it has done for every year you have read Vietnamese. Remove the letters and your ears finally get to do the work.