"The reality with Vietnam today has taught us a lesson, that no game would be easy at Asian Cup," Moriyasu said at the press conference after the match.
"It's lucky that my players have kept calm and adjusted to the situation to gain a victory."
Japan entered Al Thumama Stadium in Qatar's Doha with a slow pace. The title favorite, world's number 17 who stand 77 places above Vietnam in FIFA ranking, did not create much pressure on their opponents.
Takumi Minamino opened the score in the 11th minute following multiple shots of Japan from a corner, but Vietnam quickly equalized from Dinh Bac's stunning header, before Tuan Hai tapped in from a freekick to give Vietnam a 2-1 lead.
It took Japan until near the end of the first half to show their world-class football. Excellent shots from Minamino and Keito Nakamura beat Vietnam's keeper Nguyen Filip.
Japan had more trouble launching attacks in the second half. They brought in more stars near the end of the game, and Ayase Ueda settled a 4-2 victory for Japan in the 85th minute.
"It was very hard after Vietnam equalized and we had to made a comeback after being led," Moriyasu said, adding that they could not play pressing football as they had planned.
"We had some regrets, but the victory more or less gave us some confidence for the next games."
Japan will play Iraq next on Jan. 19, when Vietnam meet Indonesia.