Striker Sebastian Quintana burst Vietnam's bubble with a 79th-minute equaliser that enabled Qatar to salvage a 1-1 draw with the Asian Cup co-hosts on Thursday.
Vietnam were within sight of becoming the first team to reach the quarter-finals after Phan Thanh Binh's fortuitous strike after 32 minutes.
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Qatar goalkeeper Mohamed Saqr's attempt to smother Binh's speculative long-range strike went horribly wrong as he let the ball squirm under his body and trickle across the line.
But Uruguay-born Quintana, who scored in Qatar's 1-1 draw with Japan in their opening game, headed home from close range with just over 10 minutes left to throw Group B wide open.
Vietnam, who beat United Arab Emirates 2-0 at the weekend, were almost undone completely when Qatar substitute Ali Hassan hit the crossbar five minutes later.
The home side still lead Group B with four points. Holders Japan take on UAE on Friday in Hanoi.
