Schwa sound is the most commonly used sound in English language. It’s phonemic symbol is /ə/ and it is the only one sound in English language that has a name. Schwa sound is unstressed and week and pronouncing it properly makes your English to sound better. Schwa sound can be found only in unstressed syllables and any vowel sound can be pronounced like schwa.
Schwa is always associated with weak syllables. In quality it is mid (half-way between close and open) and central (half way between front and back). Most specialists describe the schwa sound as lax, not articulated with much energy. The quality of this vowel is not always the same, but in my opinion, the variation is not important.
Many, but not all, weak syllables contain schwa sound. Learners of English need to know where using schwa is appropriate and where it is not. For this we have to consider spelling (although, phonemic theory will consider it irrelevant).
Not everybody pronounces the schwa sound in the same way. American English has the tendency to eliminate the schwa sound in many situations (for example: choc(o)late, cam(e)ra, comf(o)rtable).
Comfortable/ˈkʌmftəbəl, ˈkʌmfətəbəl/ (both are considered correct pronunciation)
Chocolate /ˈtʃɔkəlɪt, ˈtʃɒkə-, ˈtʃɔklɪt, ˈtʃɒk-/ (all 4 are considered correct pronunciation)
Camera /ˈkæmərə, ˈkæmrə/ (both are considered correct pronunciation)
Some languages (not so many) have sounds that are very similar to English Schwa but Vietnamese language is not among them according to Wikipedia website. I don’t speak Vietnamese very well but I will say Schwa sound is very similar to the Vietnamese sound /ơ/.
There is a big difference difference between "ơ" and "o" sounds in Vietnamese language.
If the speaker were to pronounce a particular weak syllable as strong instead, which vowel would it be most likely to have, according to the usual rules of English spelling? Knowing this will give us a rough guide to the correct pronunciation of weak syllables.
Note: The phonemic transcription in examples below is taken from dictionary.com
Spelled with a
Barrack /ˈbær ək/
Spelled with o
Tomorrow /təˈmɔr oʊ/
Carrot /ˈkær ət/
Spelled with e
Settlement /ˈsɛt l mənt/
Postmen /ˈpoʊst mən/
Spelled with u
Autumn /ˈɔ təm/
Spelled with ous
Gracious /ˈgreɪ ʃəs/
Marvelous /ˈmɑr və ləs/