A student of mine did not know the word explain. Instead of stopping, he said:
Explain is when you don't understand, I tell you.
Perfect. That is exactly the right move.
Why this skill matters
The person in front of you will not always be Vietnamese. They might be Indian, Filipino, from anywhere at all. They do not speak Vietnamese, so you cannot switch languages to rescue yourself.
And there will be moments when you genuinely need to explain something: going through immigration, getting stuck somewhere while travelling, being misunderstood. In those moments you have to reach for everything you have and talk your way through.
Stock in a warehouse earns nothing
Vocabulary kept in your head, never spoken, never used, is stock sitting in a warehouse. However long it sits there, it generates nothing. It only has value once you take it out and use it.
It is like owning a property and never letting it. Unused, it does nothing for you.
Plenty of people study hard and have a large vocabulary, then freeze when the moment comes. The problem is not a shortage of words. The problem is that the words have never once been taken out and used.