As with many countries in SE Asia, Vietnam recieves many tourists every year and a high amount of 'backpackers'. Vietnam is a popular destination for many TEFL English teachers. With it's beautiful coastline and beaches, cheap living, warm weather and friendly locals, it is considered an ideal location to teach English. The study of English is very popular with middle and upper class families.
There basically 2 types of employment for the ESL teachers. One of these types of school, hire mainly the backpacker crowd, many of these private English language schools are located in Ho Chi Minh City. You will have trouble getting a proper visa supplied by the school and pay will generally be less. The more established private schools, require teachers to have TEFL/TESOL/CELTA certification.
Teachers work around 20-30 hours per week in Vietnam, with the monthly salary around 12-28 million VND ($755-1,760 USD). You should be able to save quite a lot with living expenses in Vietnam very low, around 2-3.5 million VND.