Two out of every three "informed Americans" fear that the situation in El Salvador will develop into "another Vietnam," with about equal numbers saying it is "very likely" or "fairly likely" that the United States will become more deeply involved as time goes on.

The Gallup survey, completed March 15, also reveals that only 2 percent of the public thinks the United States should send troops to help the government of El Salvador. Less than one in five mentioned providing economic assistance (19 percent), military supplies (16 percent) or military advisers (18 percent).

The United States is officially supporting the current Salvadorean regime of President Jose Napoleon Duarte, providing substantial economic aid as well as military supplies and advisers. According to testimony by Secretary of State Alexander M. Haig Jr. on Capitol Hill last week, El Salvador is the second of four countries on a communist "hit list" of Central American nations targeted for subversion.

Survey respondents were first asked whether they had heard or read about the situation in El Salvador and whether they knew which side the United States is officially supporting. A total of 63 percent both had heard or read about it and were informed about the official U.S. position.