It depends on the relationship between the man and the woman. The man should call the woman "cô" for the first time they meet each other. ( the woman still feel herself young!), he also address her with her name EX: Chào Cô or Chào Mai ( Mai is her name) The man should use Em to call a woman when the woman is younger than he is, or when he wants to express his friendliness...

you should call her is "Em" !^^

I think you should call her name or his name

i think the man can call woman " Em" bcz it is polite.

As you said, to be polite, the man call the woman who the same age is Chi, but most of woman want to be called " Em" to see that they are still young. Using Chi and say yourself is Toi and using Em and say Anh,, it is a perfect way which not only modest but also very friendly.