Because they are working from different models. Many Vietnamese parents grew up with an understanding of family as a shared, lifelong obligation in both directions. Many American-born children grow up with a model that values independence and treats adult family involvement as chosen rather than assumed.
Neither model is selfish. The traditional model assumes that parents sacrifice enormously for children and that children later care for parents — financially, practically, and by staying close. In that frame, moving far away, declining to send money, or making a major decision without consulting family is not independence. It is a break in the arrangement.
The American frame assumes adults are separate units. Success means self-sufficiency. Love is expressed through respect for each other's autonomy. In that frame, a parent weighing in on a career or a partner reads as control.
Migration sharpens all of this. Parents who gave up profession, language, and country for their children often carry an unspoken expectation that the sacrifice will be honored. Children raised in the new country absorb its norms by design — that was, after all, the point of coming.
What usually helps is not winning the argument but naming the two frames out loud. A lot of families discover the conflict was never about the specific decision.
Varies widely by family, generation, class, religion, and how long the family has been in the country. Plenty of families navigate this comfortably.