I've biked in Laos, Thailand, and Malaysia, but not in Vietnam. A cyclist I rode with in Laos had toured in Vietnam and he said the same thing. He also said that the Mekong delta was more pleasant for biking than other parts of Vietnam because it was much calmer and there's little motor vehicle traffic. You have to frequently cross the branches of the river on small boats. Another cyclist I met told me the same thing about Vietnam, and added that people were more aggressive than in other SE Asian countries.