U.S. AND VIETNAM ARE SAID TO AGREE ON NORMAL TRADE
United States and Vietnamese officials meeting in Washington reportedly complete sweeping trade agreement that would allow generally unfettered commerce between the two nations for first time since end of Vietnam War; agreement, which comes after four years of fitful negotiations, is expected to be signed within a week; but Congress will still have to approve agreement and extend normal trade privileges to Vietnam, a vote that some Clinton administration officials believe is unlikely this election year (M)