If you are thinking "I'll pick English up once I'm over there", I would like you to drop that idea.
Follow me through a morning
Today I went into the city. Left the car and took the train.
Into the station: tap the card. Out of the station: tap the card.
Not one word to anybody. No ticket bought from a person.
People tell me they manage fine
Someone commented that his English is not good, that he lives here perfectly normally, and that he will learn gradually.
That may well have been true some years ago.
Today, a person from Vietnam, India or Pakistan can live here for weeks and never once speak the local language.
Why
If you do not know something, ask an AI assistant. If you do not know something, search for it, and the information exists in both Vietnamese and English, more completely than ever.
Britain also signposts extremely well. Everything is labelled clearly enough that you need not ask anyone. Staff numbers have been cut, precisely because so few people need to ask.
Where that leaves you
"Go over and you will get good" was true in the era when you had to speak in order to buy a train ticket, ask directions, get anything done.
That compulsion has gone. And once it is gone, you have to be deliberate. Being deliberate takes preparation, which does not happen by itself.