The first goal of Vietnam was scored right in the third minute of the first half, while the others were made in the second.
Myanmar shortened the score in the 94th minute.
With the victory, U19 Vietnam have maintained their leading position in Group A, followed by Thailand.
To get a ticket to the semifinals, they need to win Thailand in the final match of the group on July 10.
The regional event have attracted the participation of 11 teams who were divided into two groups, competing in a round-robin format. The first and second teams in each group win the right to the semi-finals.
The tournament will finish on July 15.