The Vietnamese never let up attacking the Malaysian defence throughout and deserved the win.
In the second minute Malaysia's D. Saarvindran had his free-kick was well saved by Vietnam goalkeeper Pho Minh Long.
Vietnam had a goal disallowed in the 10th minute when the ball found Vo Huy Tuan after an exchange of passes and he shot past Farhan Abu Bakar to score, but it was adjudged to be offside.
In the 15th minute Vietnam were awarded a penalty for a foul by Adam Nor Azlin. Hong Quan Mac scored from the penalty to make it 1-0.
Just before half-time Vietnam scored when Cong Phuong Nguyen scored to make the score 2-0.
The Vietnamese increased their lead straight after the start of the second half when Farhan Abu Bakar fouled Phuong Nguyen in the box. The spot-kick was taken and scored by Vo.
Five minutes later Vietnam were awarded a free-kick, Phuong Nguyen took the kick curing it over the wall to score past Farhan to make the score 4-0.
In the 77th minute Malaysia pulled a goal back when Minh Long blocked a shot and the ball fell to Syahrul Azwari to score.
Vietnam made it 5-1 a minute later when Nguyen beat the offside trap and lobbed the ball over Farhan to increase the lead.
Malaysia face Thailand on Thursday and Vietnam will also be in action against Laos.