Plenty of people have been in the UK for a decade, working the whole time, and still manage only good morning, how are you and how much is it. More bluntly, plenty of people study abroad for three or four years and come home with disappointing English.
Why this happens
You can go a day, a week, a month in London without speaking a word of English and be perfectly fine. You tap in on the train and the bus, you scan your own shopping, you have the internet so you are on calls to friends back home all evening, and a Vietnamese landlord completes the set.
Students do not fare much better. A lecture hall of forty or fifty, the lecturer talks, the hour ends, everyone leaves. Classmates have jobs and go home afterwards, so the chances to talk are few, and usually limited to where are you from, oh nice, that's interesting.
Some people take a part-time job somewhere with foreign colleagues. After a few months or a few years you will have the till phrases down, what can I get you, that sort of thing. But English for professional work, for conversation and presenting, has not moved at all.
What actually decides it
Whether you get good at English now has nothing to do with where you live and everything to do with your attitude. Someone in Saigon or Hanoi working in an English-speaking environment, in English meetings eight hours a day, will outpace someone in London who spends every day with Vietnamese friends at a Vietnamese company.
Wherever you are, it comes down to two simple things: study and practice.
Study. Vocabulary, grammar, ways of expressing yourself, new pronunciation. The best approach is to keep working through books and materials exactly as you did back home when you were pushing for the IELTS score to get abroad. If you want to learn from local speakers, you will mostly have to notice and make notes yourself, because nobody explains anything to you in daily life. You have spoken Vietnamese your whole life. Could you explain its grammar?
Practice. An English-speaking country gives you more opportunities, but you have to take them. Before everything got so convenient, people had to speak English to get through the day, so they practised constantly whether they wanted to or not. That is no longer true.
Each day or each week, pick a few words, structures or pronunciations and deliberately use them when you get the chance to talk to an English speaker. They do not have to be a native speaker, though locals will know more idiom and regional usage. Build up gradually. One or two words a day is enough. After about a month you will notice the difference yourself.