Firstly, not all asians have tiny eyes or "slanting eyes with skin on the eyelids turned so much outwards that the eye will hardly open"... malaysia, philippines, indonesia, india and other parts of asia don't all have tiny eyes. but your friend is Vietnamese so she falls under that category...now don't laugh...she actually belongs to the MONGOLOID race. (check the source link)
these people such race usually live in the northern part of asia and their eyes are designed to fit the location. it's quite cold and there are a lot of mountains so they don't need big eyes that set deeps cause they don't have to see to far from where they live. besides, in the really really older times ('like superpast several thousands of years ago') they are one of the societies who like to stay in groups so they look out for each other. that way they don;t hve to look out far for their enemies outside their walls.