Riedl, in his third spell as Vietnam coach, put his young side through their paces in the shadow of the My Dinh National Stadium on Wednesday, while Bruno Metsu's UAE also trained in torrential rain on an adjacent pitch in Hanoi.

Vietnam, who are co-hosting the event alongside Malaysia, Thailand and Indonesia, will also face two-time defending champions Japan and 2006 Asian Games gold medalists Qatar in the group stage.

"There is not one easy group in the tournament, but our group is a little bit tougher because we have the Asian Games champion, the Gulf Cup champion and Japan and it is going to be very difficult for us. But all we can do is try," the Austrian Riedl said to PA Sport.

"The first game is the decision game already. Whoever loses will be out. This is not definite, but when you both have to play against Japan, against the favourite to win this group in my eyes, it will be hard to qualify as it will be very difficult to beat Japan. If you lose twice, then you will be out."

Riedl took over as Vietnam coach for the first time in 1998 and suffered an Association of South East Asian Nations (ASEAN) final loss to Singapore in the same year.

He was already a popular figure by the time, however, as Riedl's men won 3-0 over rivals Thailand in the semi-finals.

Riedl left in 2001 but returned two years later following a spell with Kuwaiti club Al Salmiya and he led Vietnam to a memorable 1-0 win over Korea Republic in the AFC Asian Cup qualifiers.

After a short spell with Palestine in 2005, he returned and has signed a contract which will see him through this year's South East Asian (SEA) Games and the Asian Olympic football qualifiers for Beijing.

"We are trying to prepare for the first match. We are working on what to do with the ball and also importantly what each player has to do when they lose the ball," he added.

"Of course in training, you can never simulate exactly what would happen in a match, but you can tell the player how they should move. When they move fast and in the right areas, you make it difficult for the opponent when they have the ball. We are progressing, but I don't know if that is enough for the match."

Vietnam have twice tasted victory at the My Dinh National Stadium in recent weeks, beating Jamaica and fellow continental competitors Bahrain.

Riedl played down the wins by his youthful team, which includes 12 players from the under-23 Olympic side.

"Jamaica came with a second string side and Bahrain did not deserve to lose the match because their goalkeeper made two bad mistakes," he said. "The match should have ended in a draw, maybe 3-3."