The Korean side had no difficulties in their thrashing of Hanoi, but Yong-soo is concerned about their next AFC Champions League match against Guangzhou Evergrande, saying his side can't afford to be complacent.
A Jung Jo-gook brace along with goals from Yun Il-lok, Everton Santos, Sergio Escudero, Lee Seok-hyun and Koh Myong-jin, saw the hosts send an emphatic message about their AFC Champions League intentions.
He said: "We prepared really hard at the winter camp and it paid off.
"The important thing is we have to forget about the match today and we need to prepare for the next game.
"We have an away match against Guangzhou and I know they are really strong team. They have a strong squad, however, in football, we don't know anything until the final whistle."
Hanoi T&T coach Thanh Hung was philosophical about his side's defeat, saying: "FC Seoul is a strong team. I want to say congratulations and also good luck in the group stage. The atmosphere in Seoul was really good today and Seoul performed really well."