In England it is compulsory education from 5 to 16 years old, and if you aim for going to university you need to take a common exam called A-Level over the next two years, or take international test International Baccalaureate (International Baccalaureate IB). Qualification of A-Level or IB is the condition of basic university application, and it is not possible to enter the UK university directly with just Vietnamese high school educational background alone. Students who graduated from high school in Vietnam generally take Foundation Course to strengthen the foundation of general education for one year and proceed to University afterwards.
Generally speaking, the English proficiency required for enrollment in a bachelor degree program is IELTS exam 6.0 or higher. In the case of the foundation course, it is acceptable if your IELTS is 4.5 to 5.5, but it is still better to improve your English level. Many British universities do not accept TOEFL or TOEIC as proof of English proficiency, please consider IELTS test if you wish to go on to higher education in the UK.