Your question is a little bit complicated when it applies to the reality of Vietnamese social phenomena, I think many people would find your question silly, but I am sure that your question is rooted in reasonable interaction with Vietnamese people who are so irreligious, hence you have a legitimate clue to ask such question.
Before answering your question, I would like to clarify to those who may find your question silly , to clarify to them that your question is legitimate and it applies to the reality of Vietnamese people, my clarity is the following: for if you thinks Vietnamese people is religious simply because of their temple/church visit(which however are neither massive nor frequent) you may dismisses the fact that, most do so simply due to superposition, some are sincere but mostly they are brought to these places by friends and relatives, and yet they and these people who brought them to there, they offer their worship simply due to superstitious motivation, they are far away from being a devotee , or somebody who is willing to learn the religious doctrines, or theological matters and so on. Hence, they are by large indeed very much irreligious when they attend religious services, it’s more of superstitious motivated+schedule wise it is already planned(if you are already in theses places, what else could you do apart from performing your functional “worship”?)
To answer your question itself, why is Vietnamese so irreligious, I think it has a lot to do with the individualism of Vietnamese culture, and this is a very unique plus radical element of Vietnamese culture, which’s the emphasis on “independence”, unlike other Asian cultures which you can find the strong sense of collectivism, individualism is unique to Vietnamese culture, the unique kind of “nationalism” in Vietnamese culture is taught differently as compared to what we thought about nationalism in the West, it isn’t an ideology which requires the individual to sacrifice himself to serves the nation, but rather, it emphasizes “independence” from an individual being an independent subject, then to the entire nation herself being an independent sovereign state(no foreign ruling), it’s a notion of being independent, hence it’s nationalism is based on individualism rather to favor collectivist interested over an individual one. Due to this strong sense of individualism, an Vietnamese person would be unlikely having an interest of fulfillment toward stuff such as ,worship,devotion and royalty etc. these stuff would rather be treated as useful norms, they are not natural to the Vietnamese personality; well known religions on the other hand, serves the interested of these stuff, hence, they cannot easily gains market among Vietnamese people, still they can, but it would be more hard than normal basis. Having says that, there is one indigenous religion of Vietnam does emphasizes individualism , Hòa Hảo or the so called”Protestant Buddhism”, it gained market at the initial stage , I think it had a lot to do with its doctrinal compatibility to individualism, it teaches that one can practice Buddhism and attain Buddha hood simply by practicing Buddhism at home, but it is still way distance from general civilians, yet, massive of people were willing to follow it, which even alarms the communist revolutionist at that time, and it is said that the founder of this religious sect of Buddhism was assassinated by the communist, because the communist was able to views this religions as a potential political opponent to communism.
In short, it’s the unique version of individualism in Vietnamese culture that’s making it hard for religions to gain market, unless a religious doctrine is more individualistic than the major worldwide religions, Viet Nam version of communism also gain it’s publicity due to individualistic fashion , in my view. However, one should keep in mind that, Vietnamese people are the same like other human, when circumstance is enough to qualify a massive desire Of pursuit that can only be fulfilled by a religious Doctrine, an individual must engage in it to seeks for his or her fulfillment, in other word, there must be also many religious people among Vietnamese, and you can find them while visiting religious places, hence, don’t be confused that though religions can’t hardly gain market among the Vietnamese folks, but they do have market among them.