These schools are the path of least resistance for colleagues who want to promote their institutions to overseas-bound students, including Vietnamese and expats. For example, they tend to have guidance counselors who are fluent in English, which facilitates communication and there is little to no bureaucratic red tape associated with a visit.
The reality, however, is that most of the students in Viet Nam who are planning to study overseas are Vietnamese enrolled in local public and private schools. I would estimate that the national breakdown is 90% or more from Vietnamese schools. (This is just an educated guess.)
Access to Vietnamese schools is more problematic, in some cities more than others, because of local rules and regulations. Foreigners need a permit and schools have been inundated with requests from colleagues and education companies, all of whom are promoting institutions and programs.
Since the schools’ primary mission is education, outside visits are a much lower priority in terms of staff resources and valuable teaching time. Unless you know someone at a particular school, it’s very difficult to simply send someone you don’t know (and who doesn’t know you) an email and expect a positive outcome yet alone a response.
In conclusion, while it’s worth visiting selected international schools, after determining your institution has what their students are looking for, e.g., many welcome the more selective schools, for example, you shouldn’t put too many of your outreach eggs in the international school basket, simply because they’re easier to gain access to. It could end up being a waste of your precious time and travel/marketing funds.