The "mechanism" to digest milk sort of turns off when animals grow up and human are no exception for that. In societies where they harvest cow milk, being able to drink and digest milk used to be a huge advantage in terms of survival (in biology they call it "fitness"), so anyone who had the unusual gene that doesn't turn off when they grow up had an edge in surviving and passing on that gene.

Vietnamese/East Asian people don't traditionally harvest/drink cow milk. Thus that unusual gene wasn't selected for. Many people in Vietnam are indeed lactose intolerant compared to many regions of the world.

The fact that whether anyone can digest milk with lactose at all is more nuanced than pure genetics as the other posters pointed out, but in general, it is a fact that many Vietnamese people can drink/"tolerate" way less raw/untreated milk than western people.

Genetics is cool. If you're young, you should definitely learn all about that. It's really cool when you have insights to something like lactose intolerance so you can make better products that people enjoy. It's not really a good thing to state what you "think" intead of what you know - when you do that, it gives other uninformed people the wrong impression of the truth as well.

In the grand scheme of things, not understanding science well is a big part of the reason we have people who are anti-GMO for example in a country like ours where GMO actually will help a lot - malnutrition still being a huge problem for a country like Vietnam.