The 2008 and 2018 winners made light work of the home side in their opening Group B match in this year's Southeast Asian championship to top the early standings ahead of Malaysia.
The Malaysians, coached by South Korea's Kim Pan-gon, also made a winning start, defeating Myanmar 1-0 in Yangon.
Faisal Halim put Malaysia in front in the 52nd minute when he latched on to Safawi Rashid's through ball and goalkeeper Syihan Hazmi kept out Win Naing Tun's penalty deep into stoppage time to secure the three points.
Vietnam had few issues in the Laotian capital, with Nguyen Tien Linh giving the visitors the lead in the 15th minute with Do Hung Dung doubling their advantage two minutes before the break.
The two-time winners' quality told in the second half as further goals from Ho Tan Tai, Doan Van Hau, Nguyen Van Toan and Vu Van Thanh secured a comfortable win.
Laos next take on Malaysia on December 24 while Vietnam will play three days later, also against the Malaysians in Group B, which also features Singapore.
The group phase of the Asean Championship continues until January 3 with the top two teams advancing to the semi-finals, which will be played on January 6 and 9 on a home-and-away basis.
The final will be held over two legs on January 13 and 16.
