Nguyen Viet Thang and Pham Thanh Luong came off the bench to net vital goals as Vietnam eased into the last four of the AFF Suzuki Cup with victory against Laos at the Surakul Stadium in Phuket.
The Vietnamese were frustrated time and again before Thang finally broke the deadlock three minutes into the second half.
Luong fired home a superb second just after the hour and Huynh Quang Thanh and Phan Thanh Binh added further goals to send Vietnam through to a semi-final clash with defending champions Singapore.
Vietnam entered the Group B match knowing they only needed a draw to progress following Malaysia's 3-0 loss to group winners Thailand in an earlier match.
They dominated proceedings but were unable to find the net in a frustrating first half.
Le Cong Vinh stabbed an attempt narrowly wide of the post after eight minutes and shortly afterwards, Thach Bao Khanh headed a cross by skipper Nguyen Minh Phuong against the Laotian crossbar.
Vinh suffered more frustration in the 24th minute when the ball dropped to him 10 yards from goal after a corner, only for goalkeeper Toulakham Sitthidaphone to make a brilliant reflex save to deny him.
Thanh Binh got to the rebound but fired across the face of goal.
Laos offered little threat but Vietnam keeper Duong Hong Son was forced into a diving stop in the 37th minute to keep out Phayvanh Lounglath's free-kick from 20 yards.
Thang came on to replace Vinh at the break and made an almost immediate impact, breaking the deadlock shortly after the restart.
Nguyen Vu Phong played the ball in from the right flank to the substitute, who escaped the attentions of Kitsada Thongkhen before firing his shot past Toulakham at the near post.
Thang nearly set up a second two minutes later, centering the ball to Bao Khanh, who slid in just too late to deflect the ball into the Laotian net.
Khanh was injured in the incident and replaced by Luong and the substitute quickly made his mark by making it 2-0 in the 63rd minute, bursting down the right flank and stepping inside the challenge of Kovanh Namthavixay before lashing a fierce drive into the net.
Vietnam's third arrived three minutes later as a cross from the right by Minh Phuong was glanced home by Thanh.
Binh completed the scoring nine minutes from time, tucking the ball away after Toulakham had parried a shot by Vu Phong into the air.