(FORTUNE Magazine) – What if every one of Vietnam's 72 million people bought a pair of shoes? That's today's version of a question that used to be asked about China. No wonder U.S. companies are rushing to do business in Vietnam now that President Clinton has lifted the 19-year-old embargo. Says Tom Ehrgood, international trade counsel for Digital Equipment: ''We see concrete needs in Vietnam -- PCs, efficient information systems, training, and software.'' Digital, BankAmerica, Caterpillar, IBM, General Electric, Motorola, Philip Morris, and 27 others that had already established representative offices in Vietnam seem to have an early lead. Even more companies, including Boeing, which wants to replace Vietnam Airlines' Russian fleet, are positioned to do business there. The Big Three automakers are considering assembly plants, tapping into Vietnam's educated work force. The multibillion-dollar prize: rebuilding Vietnam's roads, power lines, ports, airports, schools, and other components of its infrastructure. Consider Highway 1, the country's main north-south vein, which suffers from decades of neglect. Says William Agee, CEO of Morrison Knudsen, a major contractor in Vietnam from 1962 to 1972: ''We're excited about returning to a country where we're known as a can-do company.'' Meanwhile, consumer-goods companies, including CPC International, maker of Hellmann's mayo, Skippy peanut butter, and Knorr soups, hope to find retail customers. A war has already broken out between Pepsi and Coke, and skirmishes between Seagram's and other distillers are sure to follow. Still, U.S. companies have a lot of catching up to do. Rivals from Australia, France, Taiwan, Hong Kong, and, increasingly, Japan have a head start in selling consumer products, building hotels, assembling cars and televisions, and drilling for oil. A hidden U.S. advantage: the million or so Vietnamese who have settled in America. Many have already invested in small businesses in the South, and more are sure to do so now that it is legal.