Correct me if im wrong, but I've been told people just say "bye" most of the time. Nothing against learning "tạm biet" just wanted to hear if others agree?
You're right. In daily conversations, people rarely say "Tạm biệt" or "Hẹn gặp lại" because these sound very formal. Instead, they say "Tôi đi nhé/nha/nghe!" (nearly means "I go!") and then "bai" or "bái bai" (from "bye" and "bye bye").
I find "dạ, con đi" or "tôi đi nhé" (I'm leaving) is best because in a way you both inform the other person you're leaving and asking their permission for you to leave. So it's more polite.