No particular vaccination is required to enter Vietnam, Cambodia or Laos. However, we suggest you to be up to date regarding vaccinations such as against DTP, typhoid, hepatitis A and hepatitis B. For rabies and Japanese encephalitis, vaccines are recommended for stays in rural areas exceeding 1 month.
In Vietnam, the risk of malaria is very low in urban areas. There is no risk of malaria in big cities or in coastal towns, which means that the risk to be infected by malaria is close to zero in places concerned by conventional tourist routes going from Hanoi to Ho Chi Minh City and passing through Hue, Ha Long Bay and the Red River Delta. The most affected areas are Kon Tum, Gia Lai and Dak Lak in the Center and the provinces of Bac Lieu and Cau Mau in the South.
In fact, for rural areas, it is wise to plan for an antimalarial treatment and to bring repellent lotions while ensuring to follow the preventive measures against mosquito bites: use of repellents, protective clothes, mosquito nets (which are effective against viruses transmitted by insects, most particularly against dengue fever and Japanese encephalitis).
Please refer to the Institut Pasteur website (available in French and English) which could be useful: http://www.pasteur.fr/en.
For Cambodia and Laos, antimalarial treatment is advised, although this disease affects mostly rural areas.