I's hard to say about the time that you should start to learn Vietnamese. I think it's pretty hard so you can learn 3 months before travel here. It maybe enough for travelling. It complete depends on you.

Yes, I agree with @jack most of the Vietnamese teenagers can speak English, so you don’t have to worry. Before traveling you should collect all the information about Vietnam. In this process, StudyCountry can help you a lot.

If you want to learn Vietnamese language for travelling to Viet Nam, I think you don't need to do that, because almost Vietnamese teenagers can speak English. Just learing some simple sentences by Vietnamese language such as: Hello, Thank you, How are you?,.. it will help you are more friendly and people will love you so much.

Vietnamese is very difficult to learn so if you want to go on a trip, you can start to learn about 3 months. I can give you a hand if you need. Hope you have a nice trip

I think every time