Its amazing how many people who run schools think that you must speak American English,rofpml.
eodmatt and I were simply having a good-natured back and forth but since you bring it up, here goes.Â
First anecdotally: A Vietnamese English school teacher once commented to me that she did not see why they were expected to teach British English and use British books when "We all want to speak with an American accent." On another occasion, a parent waiting for his son to come out of a Saturday class told me that he preferred Filipino to British teachers because they spoke with an American accent. This may be disputable but their accent is much closer to US than UK.
Second: Where do their relatives live statistically? 1,799632 Viet Kieu, 41% of the world total, live in the US. That climbs to 59% if one excludes ones living in other ASEAN countries. A total of 1,979,757 live in the US or Canada while 219,723 live in Britain, Australia or New Zealand. That is a 9:1 ratio. Quite simply they want to speak the way that their cousins speak.Â
Third: Vietnamese trade with the US is more than 6 times that with the UK and about 4 time that with Australia. The foreign businessmen that they have contact with speak with US accents.
Fourth: There is an overwhelming effect of US culture (no matter how low brow.) My most fluent public school student told me that she had never attended a language academy on weekends but had learned from the Disney Channel.  Her accent definitely reflected that fact, for better or worse. Look at the choices on your TV. BBC World and Premier League football may be the only places to hear a British accent.
I have given prior thought to why British teaching, and Cambridge in particular, are so predominant not just in Vietnam but worldwide. In part it may be that the British have been at it longer and in more places.  Go back a century and teaching English was part of a colonial effort to "civilize." There was a similar conscious effort by the US to teach English in the Philippines and reverse Spanish influence. The despicable phrase "white man's burden" was coined by an Englishman, Rudyard Kipling, but it was in reference to role of the US in the Philippines. Another factor is that British focus is on TEFL or teaching English as a foreign language outside of English speaking countries, while the American focus and books seem to be mostly oriented toward TESOL, or English for speakers of other languages. Many US books include exercises based on the US immigrant experience, and are not suitable for use overseas.Â
Interestingly, only 3% of the population of Great Britain speaks with Received Pronunciation which is the scholarly term for the standard pronunciation as spoken in the south of England. I guess almost all of us are speaking some other corrupt variant.
My apologies to the OP for taking us so far  