Review the English alphabet with students until they can recite it on their own. Have students pronounce the sounds of each letter. Pay special notice to sounds of letters with which Vietnamese students often have trouble, including R, I, E, G , J, H, K, Q, W, X and Y.
Lead students through drills that emphasize sounding out words. Write short words on the board such as "dog" and then pronounce them and have students repeat the word back to you. This will help students become familiar with the construction of English words.
Use pictures and videos to help provide context to vocabulary. For instance, show a photograph of a bird and have students identify it in English. Many students are visual learners and will appreciate such aids.
Use English-language music and pop-culture in class. Music and movies are an engaging way to expose students to English. Provide a Vietnamese translation of lyrics at first or allow students to watch videos with subtitles. Rely on translation less as students' abilities progress.
Hold periodic tests and quizzes. This will help measure students' progress and identify trouble areas. Offer additional one-on-one help if students continue to struggle with the material.