Defender Bui Hoang Viet Anh netted the only goal of the derby match.
Skipper Nguyen Van Quyet’s cross from the right found Geovane Magno, who passed the ball to Anh in the box.
The defender easily scored with a left-footed kick in the 74th minute.
Hanoi had at least two more chances but both Bruno Cantanhede and Geovane failed to convert when facing Viettel’s goalie Nguyen Manh.
Viettel, who are the 2020 V-League 1 champions, created few chances as Hanoi have proved to be their nemesis in the past encounters.
Their rare scoring opportunity came in the 34th minute, when Pedro Paulo shot the ball narrowly over the bar.
Hanoi have won the Super Cup for four times, in 2010, 2018, 2019, and 2020.
Song Lam Nghe An FC and Binh Duong FC have also claimed four such titles.
Viettel FC are backed by military-owned telco Viettel.
They were promoted from the V-League 2 to the V-League 1 in 2019.
The club ranked sixth in the 2019 V-League 1 before winning the top-flight tournament last season.
Hanoi were named the champions of the V-League 1 in 2010, 2013, 2016, 2018, and 2019.