The hosts blew the southern side away in a six-minute spell in the first half when they scored three goals, with striker Venancio Caique opening the scoring in the 13th minute.
From Hồ Khắc Ngọc's free-kick, the ball reached Nguyễn Hoàng Đức who squared for the Brazilian striker to sweep in an easy finish.
In the 16th minute, Ngọc doubled the advantage after breaking the offside trap thanks to a sublime pass from Nguyễn Trọng Hoàng, allowing him to put the ball past goalkeeper Phạm Văn Tiến.
The nightmare continued for Bình Dương about three minutes later when Bruno Cunha Cantanhede roared down to the left-wing, skipped past Trương Dũ Đạt and slotted the ball home, making the score 3-0.
The visitors fought back and narrowed the gap in the 24th minute.
Goalkeeper Trần Nguyên Mạnh fouled captain Tô Văn Vũ of Bình Dương in the penalty area and striker Youssouf Touré converted the penalty to restore some hope.
In the second half, Bình Dương threw bodies forward, but couldn't make another breakthrough.
In extra time, striker Vũ Minh Tuấn put the cherry on top with a good finish to make it 4-1 for Viettel.
Head coach Nguyễn Thanh Sơn expressed his regret that his Bình Dương side were out of the National Cup.
"For me, it is very normal to win and lose in football. The weather in Hà Nội is hot, so Bình Dương didn’t play well. Only in the first 16 minutes, we conceded three goals,” said Sơn at the press conference after the match.
"In the beginning, I had no intention of ignoring the National Cup. I determined that the cup matches were to rehearse for our return to the V.League 1. It would be good to play one or two games more. However, football is like that, Barcelona also lose. I don't mean anything, but such a strong team conceded eight goals," said Sơn.
Viettel coach Trương Việt Hoàng was satisfied with his players.
"This is a worthy victory for Viettel. We are well prepared for both the V.League 1 and the National Cup. Now the V.League 1 has many teams using foreign players, the level is not much different, Viettel can not rush to attack like in the past", said Hoàng.
Viettel will meet Quảng Ninh Coal, who beat Hồng Lĩnh Hà Tĩnh 3-2 in another quarter-final, in the semi-final on September 16. — VNS