Saka was withdrawn in the second half of a victory at the Kassam Stadium on Monday that set up a fourth-round tie at Manchester City.
Arsenal face fierce rivals Tottenham in the Premier League on Sunday and the sight of Saka limping off was a concern for the leaders.
But Gunners manager Arteta revealed the England winger is "fine" ahead of the short trip to do battle with Spurs this weekend.
Arsenal outclassed League One side Oxford following a lacklustre first-half display, Mohamed Elneny opening the scoring before the in-form Eddie Nketiah helped himself to a clinical quickfire double.
Vieira came into the starting line-up as one of seven changes made by Arteta and whipped in a brilliant free-kick for Elneny to break the deadlock with a header, then set up Nketiah for the second goal.
The midfielder has endured a frustrating start to his Gunners career due to injury, but Arteta is in no doubt he will prove to be a shrewd signing.
Arteta told ITV Sport: "He has the quality. He is a really creative player, and he can decide games in the final third. He made a difference."
Nketiah has scored four goals in as many games since the World Cup in the absence of the injured Gabriel Jesus, taking his tally for the season to seven.
Arteta said of the striker's form: "It is what we want from every player. I am delighted. You have to be scoring goals, and he is doing it."
He added: "We have only one [striker]. And to cope for that many months until Gabby's back is not easy. We have to make the most out of the players we've got."
The Gunners took the lead in the first half when William Saliba headed in from Vieira's corner, only for their hosts to level in similar circumstances just past the half-hour mark at the Jose Alvalade.
Paulinho put Sporting in front after the break with a tap-in, but Hidemasa Morita's own goal seven minutes later ultimately secured Arsenal a draw to take back to Emirates Stadium for the return leg of their Europa League last-16 tie.
The draw puts Arsenal – who have only lost at home twice all season – in a decent position ahead of the second leg, but Arteta feels they can do better, even considering the somewhat unfamiliar look to his starting XI.
"We are conceding too many simple goals," he told reporters.
"We spoke about the importance of boxes, especially in a competition where you're in or out. [On Thursday], we didn't defend that well enough, and we have to be better at threatening the opponents as well.
"But of course, you have an opponent there. We've played some really emotional games recently.
"We made a lot of changes and sometimes that takes time to get that cohesion, and you could feel it in the first 15 minutes that it needed some time to adapt and click.
"If we don't expose them, though, that's never going to happen."
He added: "When you concede two poor goals like we did away from home in Europe, it's very difficult to get a positive result.
"The game had different phases because we gave too many simple balls away in our own half and gave them the capacity to [exploit] transition moments, which they are good at.
"In other moments we had total control of the game and we lost a little bit of threat, especially with the four players we are missing in our frontline.
"There were some positives because we showed a lot of resilience to get back into the game, but we need to defend our box much better and be better on Sunday."
Vieira's performance was a surely one of those positives.
The Portugal international was given only his 13th start of the season, but his delivery for Saliba's opener was his sixth assist already – only Bukayo Saka (nine) has more for the Gunners this term, and he has played almost 1,600 minutes more than Vieira.
Arteta clearly values the former Porto midfielder.
"I'm really happy with him," he added. "I think he deserves more minutes than he's had, especially in the last two months.
"Obviously he had a difficult start because he was injured for two months after a surgery, and wasn't at the level that we want.
"You saw tonight he was probably our most dangerous player. He was always making things happen. He's a player that I absolutely love."
