The Vietnamese players scored three goals in the first half and the fourth goal in the second half of the match.
The same day, the two rivals of the same group - U22 Thailand and Indonesia – drew 1-1.
U22 Vietnam are scheduled to play against U22 Cambodia on August 17.
Vietnam, a five-time silver medalist, have never won the coveted SEA Games football gold. They were drawn in Group B, considered as the "group of death", along with Thailand, Indonesia, Cambodia, Timor Lester, and the Philippines.
Vietnamese coach Nguyen Huu Thang has affirmed that his team were determined to win the SEA Games title this year.