University Foundation program is the foundation course/preparatory course to improve academic ability and knowledge for students who wish to go to University in the UK. The course is mainly for Internation students who just graudate from High school or University in their home country. 
For Bachelor degree: native students graduate High School at the age of 16 and continue their A-level for 2 more years if they wish to enter University. Hence, students in the UK and Vietnam are going to University at the same age. International students can study A-level for University Entrance Exam (UEE) depends on University's requirements or students' interests. Most of UK Universuty only requires International students to complete Foundation course and take UEE to be qualified at Bachelor programs. Students can study Foundation at University or any certified Preparation Institues such as INTO, University-partnered colleges.
For Master program: students who graduate Bachelor degree in their home country with the equevalent program and required IELTS result can apply directly to Master program at UK University. In case of english proficiency does not meet the entrance requirement, students can take Pre-master course for 3 to 9 months before going to University.