Hi there, you can not say that "Good morning" is not used in Vietnamese. Because "Good morning" means "Chào buổi sáng", but that phrase also means "Buổi sáng tốt lành".
"Morning" means "Buổi sáng"
"good" means "tốt lành" or "tốt đẹp".
You can say it (or send messages) when you meet your friends in the morning or you also say it to greet your uncle, your aunt, etc.... in the morning. When you say "Buổi sáng tốt lành" (Good morning), it will show that you hope your friends, your relatives have a good day. It looks like the best wishes for your friends, your relatives.
they will understand you if you say that, but i mean it is not natural and the locals here never say that to the others. But when the children learn English at school, they also are taught that "good morning" means "chào buổi sáng". Funny huh ? In vietnamese, i just need to say: "chào" whenever we met, there is not a special greeting for only morning