We use it to show honour or respect for adults. When an adult ask or say something to you, you have to answer with " vâng" at first. For example,
- Con ăn bánh chưa? (Did you eat snack? - your mom said)
- Vâng, con ăn rồi. (Yes, I did. - you said)
Nothern Vietnamese use " vâng " , but Southern Vietnamese use "dạ" instead.
I hope my answer is helpful to you.