Why isn't it "không, cô ấy có không có gì" ?
The "có" between ấy and không is redundant. It's a little like saying "No, she has doesn't have (anything)" in English.
How do you delineate between the sentence meaning "has nothing" vs. "is nothing". Or is the phrase "she is nothing" meaningless in Vietnamese?
That sentence is use as a slang, it's mean she has no position in the society.
It can also be translated to "No, she is fine". It depends on the occasion.
"She is nothing" means "Cô ấy không là gì" that means she does not have any position in social, family or any group and she does not exist in people's mind.
Good question, I had the same.
Then how would one say "No, she is welcome"?