Schwa sound in English language - The most commonly used English sound is called “schwa”. Actually, it is the only one sound in English that has a name. Schwa sound is displayed in phonemic transcription as /ə/.
Schwa is an unstressed and weak sound that appears in many words. Pronouncing the sound properly will make your pronunciation to be more accurate and sound more natural. Schwa sound can be find only in unstressed syllables. Any vowel sound can be pronounced like schwa.
Note: Not everybody pronounce the schwa sound in the same way. More than that, American English has tendency to eliminate the schwa sound in many situations (for example: choc(o)late, cam(e)ra, comf(o)rtable).
Comfortable /ˈkʌmftəbəl, ˈkʌmfərtəbəl/ (both ways are considered right)
Chocolate /ˈtʃɔkəlɪt, ˈtʃɒkə-, ˈtʃɔklɪt, ˈtʃɒk-/ (all 4 ways are considered standard English)
Camera /ˈkæmərə, ˈkæmrə/ (both are OK)
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 Vietnamese sound /ơ/ (NOT /o/)
Please note the difference between ơ an o inVietnamese language.
schwa (click to hear the sound of Schwa)