(FORTUNE Magazine) – Nearly 15 years have passed since the fall of Saigon and the reunification of Vietnam under Hanoi's Communist rule. But U.S. policymakers still see Vietnam as an ''enemy'' and continue to ban U.S. companies from doing business there. Many American corporations -- including oil producers, telecommunications firms, hotel operators, and consumer products companies -- worry that competitors from other nations are thereby gaining an edge. Says Robert Martin, Colgate-Palmolive's managing director in Bangkok: ''There is nothing magical about Vietnam as far as short-term profits go. But the first into a category or a country tends to hold its market share as the market grows.'' Martin believes that Unilever, his Anglo-Dutch rival, is gaining an early lead. Though no stampede, an impressive number of companies from Thailand, Singapore, Australia, Japan, France, and Britain have descended upon Hanoi and Ho Chi Minh City, formerly Saigon, since foreign investment laws were relaxed two years ago (FORTUNE, August 1, 1988). Since then, the fragile and still extremely weak economy has strengthened somewhat. Foreign oil companies -- British Petroleum, Royal Dutch/Shell, France's Total, and Belgium's Petrofina -- are the biggest investors. A more important reason to begin normalizing relations, says Owen Nee, a Hong Kong-based lawyer with Coudert Brothers, is that the conditions the U.S. set for lifting its ban on commercial ties have largely been met. Vietnam has withdrawn its troops from Cambodia, cooperated in efforts to account for American MIAs, and allowed many former South Vietnamese prisoners to emigrate.