The company collects the surcharge because its "driver-partners" have to work in extreme weather conditions, its co-founder and CEO, Anthony Tan, told Deputy Prime Minister Le Minh Khai recently.
They get 100 percent of the amount, he said.
Khai said Grab needs to establish a sustainable relationship with the Vietnamese people and its drivers to operate over the long term in the country.
On July 6, Grab had announced the heatwave surcharge of VND3,000-5,000 (US$0.13-0.21) per trip after many localities recorded unusually high temperatures.
Customers and drivers have been complaining about a lack of transparency, saying they do not know on what basis Grab decides "hot weather" and how much drivers get of the surcharge.
Grab had not made a public clarification until now.
The Vietnam Competition and Consumer Authority too had sought an explanation, but Grab asked for time to reply.
Grab Vietnam had an accumulated loss of VND4.36 trillion as of last year.