The storm made landfall in central Vietnam late on Thursday, uprooting trees, damaging homes, and triggering power outages, before weakening as it moved inland.
Authorities warned of continuing heavy rainfall of up to 200 millimetres in central provinces from Thanh Hoa to Quang Tri.
As the storm's death toll climbed in the Philippines, officials there said another 135 people remained missing and 96 had been injured.
In Vietnam, no official casualty figures had been released but state-run Vietnam News Agency reported one fatality in Dak Lak province in a collapsed house. Photos and videos on social media showed ripped-off roofs, flooded homes, and streets littered with fallen trees and debris.
The government said it had mobilised over 268,000 soldiers for search-and-rescue operations and warned of potential floods, which could affect agriculture in the Central Highlands, Vietnam's main coffee-growing region.
Kalmaegi is the 13th typhoon to form in the South China Sea this year. Vietnam and the Philippines are highly vulnerable to tropical storms and typhoons due to their locations along the Pacific typhoon belt, regularly experiencing damage and casualties during peak storm seasons.
The Philippines' civil aviation regulator has placed all area centres and airport operations under heightened alert in preparation for another typhoon that is expected to affect parts of the country at the weekend.
