Vietnamese is correct.
The possessive form of the singular, proper noun Vietnamese is Vietnamese's.Example: The Vietnamese's passport is in order, the others' are not.Note: The noun 'Vietnamese' is a word for a person from Vietnam.The word Vietnamese is also an adjective, a word used to describe a noun.Example: He carried a Vietnamese passport.
The proper adjective for Portuguese is "Portuguese."
Mayan is itself a proper adjective.
Russian IS an adjective.
The proper adjective for Mexican is "Mexican."
Vietnamese is the proper adjective for Vietnam.
An adjective describes a noun. You could potentially describe a person or thing as "Vietnamese".
As I understand your question, you are asking how you would describe something that comes from Vietnam. And the answer would be Vietnamese. Food that comes from Vietnam would be Vietnamese food.
The possessive form of the singular, proper noun Vietnamese is Vietnamese's.Example: The Vietnamese's passport is in order, the others' are not.Note: The noun 'Vietnamese' is a word for a person from Vietnam.The word Vietnamese is also an adjective, a word used to describe a noun.Example: He carried a Vietnamese passport.
The proper adjective for southwest is "southwestern."
The proper adjective is Haitian.
The proper adjective for Portuguese is "Portuguese."
The proper adjective for George is Georgian.
Mayan is itself a proper adjective.
The proper adjective for Siam is Siamese.
The proper adjective for Georgia is Georgian.
Freudian is the proper adjective for Freud.