Vieira has a launch clause of round £42million, however the Gunners want to deliver him in for lower than that.
Experiences in Portugal have the price at round £30m upfront, with additional add-ons within the area of £4m that means it may rise sooner or later.
Nonetheless solely 22, Vieira comes with a rising repute from Portugal and he has been linked with Tottenham and Manchester United this summer season.
He has starred for Portugal at youth degree and final yr was voted the most effective participant on the European Underneath-21 Championship.
Vieira was an everyday for Porto final season and completed the marketing campaign with seven objectives to his identify.
The Gunners want to bolster their squad this summer season after narrowly lacking out on qualifying for the Champions League final season, with Tottenham pipping them to the ultimate spot.