Travelling to the larger cities in Vietnam such as Ho Chi Minh City (Saigon) and Hanoi poses very little risk of malaria, therefore antimalarial drug prophylaxis is only indicated for travellers staying overnight in (remote) rural areas of Vietnam:

CENTRAL HIGHLANDS: This includes highland areas of Kon Tum, Gia Lai, Dac Lac, Phu Yen, Khanh Hoa, Ninh Thuan and LamDong provinces (below 1200m).

SOUTH VIETNAM: Ngoc Hien district in Minh Hai province, the southern tip of the country.
The drugs of choice for prophylaxis in Vietnam are Malarone and Doxycycline (better tolerated).

In other rural areas, the risk of malaria is low, but cannot be excluded. So if fever develops within the first year of visiting these areas, malaria should be suspected.