It’s because Vietnam has only been a part of the global market for 30 years since the 1980s. Vietnamese currency stands third in the top 10 of the weakest currency in the world in 2021 according to ugwire.com.
This low currency trait of VND creates an opportunity for those who may consider living a stable life with only average income. For example, in America, most people will struggle if they make less than 500$ a month. It’s the opposite in Vietnam. With 500$/month you will be able to live a pretty decent life. The food and the rent in Vietnam are pretty cheap.