That description is not accurate. The sounds in Vietnamese and English are different, yes. (And the sounds in other European languages are
also different, not just like English.)
As for just
learning English, it's best to practice while listening to native speakers.
But to
understand how the sounds work, you should study something about how sounds are physically produced.
The vowels of
all languages are produced within the same physical space. But languages use different divisions of that space. English has more vowels (more distinctions/areas) within the mouth, and Vietnamese has fewer. (But Spanish, for example, has 5 vowels, so not all European languages have many like English.) Vowels can be described by three features: (1) height (how open the jaw is, how high the tongue is), (2) backness (how far back/forward the tongue is in the mouth), and (3) lip-rounding ("o" is a round vowel, and "e" is an unrounded vowel). To understand the location (height+backness), consider the extreme vowels a ("aaah"), i ("eeee"), u ("oooh"). a is very low in the mouth ("open"/"low"), i/u are high vowels ("closed"/"high"). And i is far forward, while u is back. You should be able to feel that with your tongue. Other vowels are just variations on that.
Consonants are more relevant to
where they are produced.
Place of articulation for consonants can go from the lips all the way back to the glottis (voice box / vocal folds). (But I'm not aware of English having more consonants farther back in the mouth than Vietnamese!) And consonants are also defined by
manner of articulation: is the airflow totally stopped (like t, d, p, k, etc.)?, is a narrow hissing sound produced (like f, s, etc.)?, is air going through the nose (like n, m, etc.)?, or is it another type entirely (like r, l, y, etc.). Consonants also distinguish
voicing, whether the throat is buzzing or not (s vs. z).
You can read more here:
https://en.wikipedia.org/wiki/International_Phonetic_AlphabetAs for the description of sounds in the "neck and mouth" or "throat and chest", I think those are just inaccurate perceptions of different speaking styles. It is a very well know fact that non-experts (non-linguists, and people who don't know much about phonetics/phonology, or the International Phonetic Alphabet) don't really do a good job of explaining what seems like a different kind of sound. For example, some accents are called "nasal" when they either actually have MORE OR LESS airflow through the nose (like the letters n and m, but for all sounds). So if someone says "your accent sounds nasal", that doesn't actually
mean anything specific. It just means "it sounds different to me".
My recommendation is to either study how sounds are produced physically (as outlined above), or work with a trained linguist or English as a Second Language teacher/tutor, so that they can give you quality advice. Note that not all English speakers working at English teachers are qualified to give good pronunciation advice. Some have backgrounds in linguistics and relevant training. Others do not. It depends on the qualifications required for the job they have, as well as their individual background.
