How do you guys tell Chinese, Japanese, Koreans, Vietnamese apart?

Just ask them directly in a polite way.

The chinese have square jaws and the vietnamese have slanty eyes.

Vietnamese are south east asians for the reason they look like south east asians. 

Once in a while you will see vietnamese that can pass as chinese but majority of vietnamese dont look east asian. 

Vietnamese are south east asian dude. 

you should do a thread about telling nordic races apart.