Teaching in Vietnamese public schools, you need to be ready for a wide range of circumstances. There may be everything from brand new schools with TV in every room that you can link to your laptop to schools with no fans and few windows. One constant will be a class size of about 50 students. You will be expected to work mostly in conversational skills while the local teacher concentrates on grammar and reading. Teaching conversation is clearly better done in relatively small groups so class size presents a challenge. You may want to work with one pair of students but you have 48 idle. Of course I stressed that they should be listening in, but that only goes so far.
Generally speaking their English level will be similar but some students may be outliers on either side as their grouping is based on testing in all subjects. You will probably have an aide who either works for your employer or is a teacher in the school. The former is preferred but still may take some management from you if they tend to over-translate. I am told that in elementary schools, the classroom teacher will stay in the room which can be great or terrible depending on how they act.
As terrible as that sounds, I loved public schools. However, I was an elementary school teacher for a very short time after college so I had a set of skills that I was able to reemploy, much like riding the bicycle. The two best public school teachers that I knew were retired teachers. One was from Texas. He had a strict routine for his high school classes that involved them keeping vocabulary lists in notebooks and weekly testing. One discipline problem arises from the fact that the students learn (students are always smarter than school administrators think) that even if you are asked to grade them on speaking, it will not go into their formal grade records. Vietnamese teachers are able to use grading as a cudgel and they do! The other was a retired AU teacher. He did all his time at one middle school and had no language center classes by his choice. It gave him weekends and nights to relax and be with his adopted family.
I know this may seem like a dismal picture to some, but if you are a person who finds that you truly enjoy teaching and the students, you will enjoy teaching in public schools.