How does a typical Vietnamese person look like?

Im at work and a customer just came in,,she looked around, and out of no where she asked me: are you half? (meaning half Asian, and half..something else...) I said "no". Then she asked me where I was from, and I said "Vietnam". She then said: you don't look vietnamese, you look half something...both of your parents are Vietnamese? I replied "yes", and she kept on.."I'M Vietnamese..YOU don't look Vietnamese..anyhow, thanks, bye!"..and then she left the store...

the incident made me think about how everyone has different conceptions of what a typical Vietnamese person looks like...or what a typical any-kind-of-nationality looks like...you see, to me, the woman who came in looked Chinese..and I, I look Vietnamese,..are you one of those people who think they can tell the difference between a Chinese, and a Vietnamese person? Can you tell the difference between a Swede/Danish person from a Norwegian?..If so,, DO tell me how you do it, cause it intrigues me how people just KNOW that I'm NOT Vietnamese, or that I just MUST be Vietnamese..