You will often hear variations in pronunciations from not only country to country, but even regions within the same country, even from native speakers. This is true of any language. That's why native speakers can often pinpoint where other speakers are from, based on those differences.
You list Vietnamese as your native language - can't you often tell if another native Vietnamese speaker is from a particular part of Vietnam based on their pronunciation or vocabulary? That's the same thing happening here in your post with English.
You could use whichever pronunciation of 'est' and 'less' you like. Neither one is "more correct" or better. My suggestion would be to adopt the pronunciation most widely used in the area you plan to live or work.