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.
The Lisbon club won two European Cups in the 1960s and last reached a European final in 1990. They have also struggled domestically, winning only four league titles in the last two decades.
Vieira said the celebrations affected preparations for the Portuguese Supercup in August, which Benfica lost 2-0 to Andre Villas' Boas refreshed Porto side.
That was followed by a dismal start to the league, a 5-0 thrashing by Porto and a weak Champions League group stage campaign that dropped them into the Europa League. Although they showed signs of recovery to win the League Cup, they could not sustain the run.
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.
