The midfielder will bring his successful temporary spell with the Eagles to an end this weekend and is set to receive a tremendous reception at Selhurst Park following a season where he has hit eight goals and provided five assists in all competitions.
Gallagher will return to Chelsea this summer and the noises coming out of both clubs suggest that is where he will remain for the start of next season, but Vieira is thrilled with how the loan went for both the player and Palace.
“Conor has been fantastic to us,” he said.
“I think he felt like he’s at home, the way he has been performing for us. He understands the way we want to play the game and he brings his energy, his determination, so hopefully he will finish with a win.
“We know that he has to go back to Chelsea and then what will happen? We don’t know.
“But what is important is that he came for a year, he worked hard, worked well and he gave something to the team and the football club.
“I think it was a win-win. I think he was really happy with us, he played really well, which allowed him to go to England. He’s happy, so we are happy.”
Gallagher was used off the bench during Thursday’s 3-2 defeat at Everton, which ended Palace’s hopes of setting a club record for most points achieved in a Premier League term.
The dramatic loss at Goodison Park – where boss Vieira was involved in an altercation after full time with a home supporter – means the Eagles can now only reach 48 points, not the 50 required to reach a new best total, but they can still claim a top-half finish.
Vieira also insists if any fanbase deserves one more victory this season it is the Palace faithful, and that is why there will be no lack of motivation against United on Sunday.
He added: “It’s going to be really easy and simple. It’s a home game and when you look at the support that we have had this year, it’s been fantastic.
“And if there are fans that deserve a really good performance, it will be our fans, so I don’t have any doubt that we will try our best to perform and try to win the game for them.”
One subplot to the fixture will be the presence of incoming United boss Erik ten Hag at Selhurst Park.
The Dutchman will not be involved in team selection but has spoken with interim manager Ralf Rangnick this week ahead of his official start day on Monday.
A constant subject of debate remains the future of 24-goal hitman Cristiano Ronaldo, but Ten Hag recently described the attacker as a “giant” in a sign his time at Old Trafford may not be over.
Vieira, who faced the Portuguese superstar as a teenager, insisted: “I think when you’re talking about Ronaldo, you’re talking about one of the legends of the game.
“I think every single year there’s always a question mark about him. But when you’re looking at his record and the number of goals that he scored, we shouldn’t question the player.”