Many teachers enjoy their experience of teaching English in Vietnam, although it isn't as laid back as Thailand, for example, and many find it a stressful experience too (the traffic being a frequent complaint). However, the population is young, and keen to learn English, as it is seen as being important in helping them to get a job. EFL jobs in Vietnam generally require less experience and tefl certification than many other countries. Most TEFL jobs in Vietnam are in Ho Chi Minh City (Saigon) and Hanoi, the capital.
Most of those teaching English in Vietnam are working in private language schools. There are also jobs available in Vietnamese universities, but they can be harder to find. Many private language schools only give part-time work to avoid paying tax, so many English teachers work at two schools. As in all countries, the quality of private language schools varies a lot. Do some research on the school before accepting a job: speak to other teachers and check online.
Not all 'volunteer' positions are really volunteer work. Do check this one out, there are many stories of overworked and underpaid 'volunteers.' You're probably better just turning up and finding work yourself.
Private language schools usually ask for a BA and TEFL certificate, but many teachers find EFL jobs in Vietnam without these. If you want to work legally with a work permit, you will need a degree. Many of the jobs advertised online require a degree, a TEFL certificates, and experience.
However, most jobs are to be found by being there and looking around yourself. For these jobs qualifications are not always required. A smart appearance is important; and for some jobs a tie is needed. Apart from Ho Chi Minh City and Hanoi, there are also opportunities for tefl jobs in Da Nang, and sometimes in other cities.
Private students are easy to find. As well as local students, you there are many Japanese and Koreans working in Vietnam, and they often want their children to learn English.
Work visas are possible, but hard to get. Many of those teaching English in Vietnam, do so on tourist visas, and renew them for three months. Many schools also change the teacher's tourist visa to a business visa. A lot of schools are not registered to get work visas.
Salaries for teaching English in Vietnam are low, but higher than local salaries (and higher than in many of the neighbouring countries, for example Thailand), so you can live well, if you live simply. Hourly rates are around $10-$20 an hour - although I've seen rates for 45 minute classes at $6. Monthly salaries range from $900-$1,800. As always, you can find examples outside the usual range.
A lot of business is in Ho Chi Minh City, and a lot of teachers work there. There are also more bars and western restaurants there. However, many teachers also like teaching English in Hanoi. There is still a French feel to parts of the city, and if you want to learn Vietnamese, it could be the better choice.
One of the good things about living in Vietnam is the opportunity for travel; both in Vietnam, and in other parts of South East Asia and the Far East. Vietnam has some beautiful places to visit, and it also has wildlife unique to the country.