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.
Peace, MAA
Here’s an abridged version of a recent interview I did with Vietcetera, a “consortium of artists, writers, designers, photographers, musicians, technologists, and business people dedicated to a fresh look at an evolving Vietnam. Vietcetera seeks to find the untold human stories of the people that are contributing to a new, modern Vietnam. From design to business to architecture to film. We want to both give a new and youthful take on Vietnam that both local and foreigners can appreciate.” 
I wrapped up a very busy week by chairing a general session about how to recruit students in Viet Nam without using an education agent. (Ideally, institutions do both in highly competitive markets like Viet Nam.) 
Finally, thanks to my distinguished colleagues, Diana Sampson (Shoreline Community College, WA) and Stephanie Sieggreen (Western Kentucky University) for their outstanding contributions. It was a pleasure and an honor to work with both of them. 