Keane and Vieira clashed prior to kick-off in the Highbury tunnel after an altercation between the Frenchman and Gary Neville, causing a furious Keane to confront the Gunners captain.
And Darren Fletcher - then a 21-year-old rookie - has explained just how things became heated, and what was going through his and his team-mates' minds.
"I heard Gary Neville come into the dressing room, saying that a few of the Arsenal lads had just started on him in the tunnel," Fletcher said to UTD Unscripted.
"It sounded surreal because that doesn’t happen. It hadn’t happened before and hasn’t happened since. It was a unique situation.
"Gaz didn’t seem too bothered by it, he just let out that laugh that he does.
"But it was game time, so Roy did what he does.
"I think it was kept among the players. He didn’t broadcast what he was about to do. It was just said to a few of the lads; it wasn’t as though Alex Ferguson had sat us down and spoke about it. Roy took it upon himself.
"So we get into the tunnel and it all kicks off.
"Suddenly, Roy Keane starts having a go at Patrick Vieira, before we’re even on the pitch.
"In that one moment, you’re in complete shock. All sorts of things run through your head.
"Is it going to kick off again, like at Old Trafford last year?
"What’s the ref going to do?
"Are they going to come to blows?
"You’re on edge, getting ready to rumble or hold somebody back. In that moment you don’t know what’s going to happen. Everything’s on a knife-edge."
Fletcher insists, however, that Keane was in complete control of the situation, and said the United skipper had the full backing of his team-mates.
"When you look back at the footage, it doesn’t look like Roy’s in control. He is.
"You saw the Arsenal players almost telling Vieira to leave it, not to get involved, whereas we didn’t need to do that with Roy. We knew Roy. We knew he’d be on it.
"We were backing Roy but letting him do his thing. He’s our captain, our leader, he would always do what was in the best interests of the team. It wasn’t Roy going in on his own, trying to make himself look good, it was just Roy defending his team-mate. That brings out the best in him.
"We were ready to back him no matter what, and none of us would ever have held him back. No chance. We knew he would be cool, calm, collected.
"So in that moment, you’re not sure which way it’s going to go, but you know you’re on the right side, however things pan out."
Eventually, the pair were separated and calmed ahead of the game, and Fletcher believes that the incident focused United and spurred them on to victory.
"When that moment passes and you’re getting ready to go out onto the pitch, you’re thinking: We’re not losing this now. No way," he told UTD Unscripted.
And despite trailing 2-1 at half-time, with Vieira on the scoresheet, the Red Devils rallied and came from behind to win 4-2, with Cristiano Ronaldo grabbing a second-half brace and John O'Shea sealing the win with a sumptuous lob.