The West Asian side's victory was never in doubt after they took an early lead through a Younis Mahmoud header, but the Iraqis made it tough going against the Vietnamese until Mahmoud added his second with a free-kick midway through the second half.

And Vieira blasted the performance, describing it as 'terrible' and saying that his players had become too complacent after taking an early lead.

"We played too individually and a lot of the players relaxed too much because we scored in the first minute," the Brazilian said.

"Vietnam is a dangerous side on the counter-attack but today they had a bad day. If their passes came off then we would have been in trouble."

While the Iraqis have yet to be beaten in this year's Asian Cup, Vieira is adamant that they must improve in order to win their semi-final on Wednesday and reach the Asian Cup final for the first time ever.

"I don't know who we will meet in the semi-final yet; it might be Iran or it might be South Korea. We have to wait and see," he added.

"But we can't make the mistakes we made today and in the game against Oman. I say to my players everyone can make a mistake but you cannot repeat them?make new ones!

"Iran and South Korea are at another level and we have to step up if we want to reach the final."

Meanwhile, Vietnam coach Alfred Riedl believes the physical difference between the two teams was a major factor in his side's defeat.

Playing in the Asian Cup for the first time since 1960, Vietnam surprisingly advanced to the knockout stage thanks to a 2-0 win over the United Arab Emirates and a 1-1 draw with Qatar during the group stage.

However, they found little joy against the Iraqis on Saturday.

"Iraq deserved the win there is no question about that but we just couldn't compete with Iraq physically," said Riedl.

"We have to work on our physique but the problem is the players are just too small.

"We are capable of competing at South East Asian level and could have occasional success in Asian matches but our players physique is too small.

"Of course we got good results against the UAE and Qatar but you cannot reach the top five with players of this size. It is much harder to compete against players that are much bigger than you. Everyone in sport knows this."