The Englishman, who left Anfield in a 49 million-pound ($76 million) deal, clinched his brace inside 31 minutes amid a flurry of forays into the box that shook Vietnam and left the hosts struggling to stem the onslaught.
Aleksandar Kolarov and David Silva both scored a goal in each half for a City side testing out youngsters in a lop-sided match before near-sellout Hanoi crowd.
The victory was City’s third in four pre-season games on the road after wins in Australia over Melbourne City and Roma and a 4-1 defeat by Real Madrid. City play VfB Stuttgart in Germany on Saturday.
Kolarov put the visitors ahead in the 11th minute, floating a 25-yard free kick into the corner with the keeper rooted to the ground.0 Sterling opened his account after 19 minutes with a superb curling shot from an angle after being teed up by Silva. Silva prodded a loose ball home from close range just moments later, after his header was pushed on to the crossbar by Vietnamese keeper To Vinh Loi.
City made it four when Sterling darted into the area and blasted low into the corner after a one-two with Nigerian striker Kelechi Iheanacho. Vietnam defended desperately, and clumsily, but stepped up a gear at four down when they started playing for pride, with good moves from Le Cong Vinh and Nguyen Van Quyet bringing to life a deflated crowd.
But the fightback was short-lived, with Kolarov converting a spot-kick six minutes after the break and Silver adding his second from close range.
Marcos Lopes slotted under the keeper after 73 minutes and Jose Angel Pozo made it eight with 12 minutes left.
Vietnam salvaged a consolation goal in the dying seconds when Quyet lobbed the ball over substitute keeper Willy Caballero.
City Manager Manuel Pellegrini said Sterling had shown why he was on city’s radar.
“You watched the way he played today, he has a lot of speed,” Pellegrini told reporters.
“Raheem is a very good player, that’s why we brought him to our team. He has a lot of things we didn’t have in our squad.”
(Editing by Ed Osmond)