Portugal now need a draw in their last match in Denmark on Tuesday to advance to the finals.
Victory also meant Bento, who took over in September 2010, became the first Portugal coach to win his first five competitive matches.
"This record is only useful to value the team's work which has brought us to the brink of qualification after a tough start. It is a record for the team not for me," he told reporters.
The home side scored with their first chance, Nani heading the ball past keeper Stefan Magnusson on 13 minutes following an inviting cross from Eliseu on the left.
Nani quickly added a second after reacting to a misplaced back-pass from Iceland captain Solvi Geir Jonsson and netting from 12 metres.
"This game was excellent for me. I scored two goals but the most important thing was the three points," said the winger.
Portugal went 3-0 up just before half-time when striker Helder Postiga converted a centre from Bruno Alves.
Iceland, without injured forward Eidur Gudjohnsen, came back strongly in the second half and pulled a goal back as forward Hallgrimur Jonasson's header gave keeper Rui Patricio no chance.
The 25-year-old Jonasson then notched his second goal after 68 minutes with a delightful back-heel.
Portugal rallied in the final 10 minutes as Eliseu set up Joao Moutinho for their fourth goal before Eliseu himself made it 5-2.
Eliseu, who plays for Malaga, stood in for Fabio Coentrao at left-back and offered the attack width and speed. He put the seal on his performance with a fantastic swerving shot from distance.
"There were a few players who were given an opportunity today. Eliseu for example had a great game and showed the coach he can count on him for the future," said Nani.
A late penalty by Gylfi Sigurdsson made the scoreline look more respectable for Iceland.
Portugal lead the group on 16 points, level with Denmark but boasting a better head-to-head record.
