That country is asking for international assistance in finding the cause and a cure.
The disease starts with a rash on the hands and feet that looks like severe burns.
The most acute cases can lead to high fever, loss of appetite and eventual organ failure.
Vietnamese officials say the disease was first detected last year in the central province of Quang Ngai and appears not to be highly contagious.
New cases had subsided until a string of new infections recently emerged, mainly in one impoverished village.
Frightened residents of Reu have laid branches across the paths to the homes of infected people in an attempt to isolate the outbreak.
Initial tests suggest the disease may be caused in part by the Rickettsia bacterium, which is transmitted by lice or flees.
Photo: VietnamNet Bridge