Twelve months after claiming a record 32nd league crown, Benfica's poor season was completed last week when they were denied a spot in the Europa League final by Portuguese rivals Braga last week.
At home, rivals Porto clinched the title and knocked Benfica out of Portuguese Cup.
"We spent too much time celebrating last year's title and all that partying distracted us from preparing the future. We have to admit to the mistakes," Vieira told the club's television channel.
Benfica were rampant last season, producing attacking displays to win their first league title in five years and leaving fans convinced the club had halted Porto's domination.
Vieira said the celebrations affected preparations for the Portuguese Supercup in August, which Benfica lost 2-0 to Andre Villas' Boas refreshed Porto side.
Despite the underwhelming season, coach Jorge Jesus received Vieira's backing.
"He deserves my confidence and that of the fans. We cannot have a short memory, he earned us some trophies. This year things did not go well for him but we have to learn lessons and make corrections," Vieira said.
