Learning Vietnamese is not so difficult,i think the most important thing you need to do is listening regularly. You will get used to it with practise. In my opinion, the most diffcult thing for all of who are learning Vnese is how to use the words exactly when you speak or write. Especially, when you speak to the other , its needed to choose the right words , build the right sentences so as to make the listeners understand your real point and make them interested in your talk.
Don't worry, no pain no gain!!!