Since you are a Chinese, there are many advantages for you in learning Vietnamese because our culture have a lot of things in common. To be fluent at daily communication, I think it's not too hard if you have the environment to practise speaking every day. In this case, maybe learn by heart is the fastest way and how long it will take to learn is depended on how many sentences you can remember per day. If there is something that I can help, feel free to contact me.