I've got a player in the vietnam U21 national team. His name is Carlos Manuel Cortez
Last I checked, you don't have a player from Vietnam.
Crotrez is from Venezuela and he isn't on the U21 team.
What kind of training are you doing? If you are training pressure 3 positions then that is the reason why he is training so slow. If you want the NT prospect you should 1 position train him