Like any English student, Vietnamese English-as-a-second-language learners should first learn the English alphabet and basics of pronunciation. Vietnamese students often have trouble pronouncing letters such as R, E and G, so teachers should pay particular attention to the pronunciation of these letters.
After students have memorized the English alphabet, move on to pronunciation and construction of basic words. At first, try to avoid words that are exceptions to pronunciation rules. For instance, "cat" would be a good word to teach beginning students, while "lamb" would not be. These lessons also introduce students to the basics of English spelling.
Students should also begin memorizing basic words and phrases. Consider showing pictures to the students and having them write the English word, and vice versa. These types of lessons mostly help with broadening the students' vocabulary.
Eventually, the Vietnamese students should move on to simple sentences and phrases. At this point, grammar is the primary lesson being taught. Students should learn about phrasing and complex sentences, and can be asked to write short essays. A good way of teaching this lesson to Vietnamese students is through the use of English language movies. At first, leave Vietnamese subtitles on to allow students to acquaint themselves with the pace and common phrasings used in the English language. Eventually, turn off the subtitles, forcing the students to use their understanding of the language and contextual clues to deduce the script.