Vietnam Airlines is reducing capacity from London over the summer before resuming a daily schedule in October.
The carrier will operate three non-stop flights a week from Heathrow to Hanoi and two to Ho Chi Minh City from April 28.
Daily frequencies by Boeing 787-9 Dreamliners return on October 26.
The national flag carrier will also start non-stop flights between Ho Chi Minh City and Denpasar in Bali in June, further increasing connectivity via Vietnam.
Vietnam Airlines offers the only non-stop flights between the UK and Vietnam, operating from Heathrow Terminal 4.
Outbound flights are scheduled to offer overnight journeys, with onward connections via Hanoi and Ho Chi Minh City to multiple destinations, including Cambodia, Laos, Singapore, Thailand, Malaysia, Indonesia, China, South Korea, Japan and Australia.