The duo replace Beto of Sevilla, who has a hip problem, and Braga's Ruben Micael, who has a knee injury.
Meanwhile, captain Cristiano Ronaldo has been causing concern with a painful thigh but his teammates were confident that the Real Madrid star would be able to play in Belfast.
"Cristiano is doing everything humanly possible to be ready to play against Northern Ireland," Beto said at the team's training camp in Obidos, north of Lisbon.
"Ronaldo is always ready to help the national team. I think he will play and we will be able to count on him," added Vieirinha, of Wolfsburg.
Portugal, who lead European qualifying Group F by two points from Russia but have played a game more, face Northern Ireland at Windsor Park before travelling to Boston in the United States for a friendly against Brazil next Tuesday.