HANOI -- After chasing off cheap Chinese beer competitors a decade ago, Vietnam's state-run brewers are gearing up to fight a new invasion from Western beer brands. One of their secret weapons is a popular, unpasteurized brew called bia hoi that already enjoys a cult following in parts of the country.

Lured by Vietnam's population of 86 million people, nearly half of them between the prime beer-drinking ages of 20 to 40 years old, a wave of foreign brewers has landed on the country's shores in recent years to capitalize on...