Arsenal did have the option to recall Fabio Vieira from his loan spell at FC Porto, but only briefly at the end of December.

James McNicholas reports for The Athletic that Arsenal had a brief window at the end of the December where they were able to recall Fabio Vieira from his loan at FC Porto. The club decided against it, and after that, the opportunity was gone.
Mikel Arteta suggested the same in January, saying: “There were certain possibilities with the Fabio situation.
“We’re exploring all of the things that we can do, just in case we cannot do them as well, how the injuries are progressing and when we’re going to have them available. So we are aware of everything.”

Perhaps Raheem Sterling nearing his return to fitness helped make up Arsenal’s mind.
Though they knew they’d be without Bukayo Saka until March, at the time Ethan Nwaneri and Gabriel Martinelli were both fit, with Sterling making his comeback just a week after the end of December. Vieira’s return probably didn’t seem so urgent.
It would be interesting to see if the club would have made the same call after Nwaneri’s injury.
Or indeed at this point in February, with the knowledge that Martinelli will be out for at least a month and that Sterling is continuing to struggle to have an impact. There’s a lot more pressure on Nwaneri to perform now.

Yet Nwaneri’s first-team involvement was part of the reason why Arsenal chose to loan Vieira to begin with. Arteta confirmed that a motivating factor behind the move was to give Nwaneri more chances on the pitch.
Those chances have ended up being slightly more enforced than the manager would have hoped, but that’s just how it goes in football sometimes.