Yes. Culture is not a language test. Language can be an important connection to heritage, but Vietnamese identity is also carried through family, history, food, memory, community, traditions, and lived experience.
This question comes up constantly, and it is usually asked by someone who already feels the answer is no — often because of a specific moment: freezing at a family party, being teased by relatives, being told they sound funny, or watching a grandparent's face when they answer in English.
It is worth separating two things. Language loss across generations is real, and it is worth working against if you want to. But fluency has never been the sole boundary of belonging, in any diaspora, and treating it as one erases a great many people: elders who spoke regional dialects, mixed families, adoptees, people raised away from Vietnamese communities, and anyone whose family circumstances made language transmission impossible.
What people carry instead is not nothing. It's knowing how a house should smell on Tết. Knowing to greet the oldest person first. Knowing which container holds the good fish sauce. Knowing your grandmother's story even if you heard it in English.
And if you do want the language, starting as an adult is completely normal. Many people find that the words come back faster than expected, because they were never fully gone.
Attitudes differ. Some families are relaxed about language; others treat it as central. Both feelings are common and both are worth taking seriously.