There are two types of teaching qualifications to choose from; TEFL (teaching English as a foreign language) and CELTA (Certificate in English language teaching to adults). Both are accepted in Vietnam, though the courses can differ in depth and content.
Generally, the CELTA is considered a higher quality of qualification and is recommended if you are a non-native English speaker to prove your English proficiency.
There are hundreds of different companies offering these qualifications which can make it confusing to pick which course, so let me share my recommendations for both online and in-person TEFL courses.