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.
Vietnamese is the proper adjective for Vietnam.
The proper adjective for Siam is Siamese.
The proper adjective for George is Georgian.
No, old is not a proper adjective. A proper adjective is an adjective derived from a proper noun, such as Swiss cheese, Bolivian pottery, Chinese silk, etc.
France is always a proper noun it is never a proper adjective (what is a proper adjective? The adjective is french. I like french food
The proper adjective for Antigua is Antiganantigan
Oklahoman is the proper adjective for Oklahoma.
Kuwaiti is the proper adjective for Kuwait.
Paraguayan is the proper adjective for Paraguay
A proper adjective is a descriptive word derived from a proper noun.Some examples are:Asia (proper noun) - Asian (proper adjective)Brazil (proper noun) - Brazilian (proper adjective)Canada (proper noun) - Canadian (proper adjective)Elizabeth I (proper noun) - Elizabethan (proper adjective)Sigmund Freud (proper noun) - Freudian (proper adjective)George Orwell (proper noun) - Orwellian (proper adjective)Proper nouns and proper adjectives are always capitalized.
The proper adjective for the proper noun Shakespeare(always capitalize a proper noun) is Shakespearean (always capitalize a proper adjective).
Arabian is a proper adjective.