The new diplomatic ties to the Vatican will give the United States little influence over Roman Catholic policy in such areas as Central America, observers here say.
''It is true that US Catholic clergy are getting more involved in Central America, for instance, but it's by no means certain that diplomatic status means (President) Reagan will have the church on his side in his Central American policy,'' a longtime academic observer commented.
The move puts an official stamp on an already existing, cordial relationship. According to a writer and scholar of Vatican diplomacy, ''The Vatican, especially under John Paul II, would like to have diplomatic relations with anyone - including Moscow - and everyone. (It) is particularly pleased with this , its first direct contact with one of the superpowers.''
Poland reportedly will join the list in the near future.
While liberal Catholics view the move to official representation with suspicion - foreseeing direct Vatican interference in American church affairs - the reaction from the US clerical hierarchy in the Vatican is that the US is the chief beneficiary.
''It is not a religious question, but a question of national politics which has been happily dealt with and resolved in this context,'' said Msgr. James Malone, president of the American Bishops' Conference in the Vatican's daily newspaper, L'Osservatore Romano.
Another cleric comments that the US can now avail itself of the Vatican's worldwide organization. Under John Paul II, the Vatican has assumed more and more negotiating credibility in foreign affairs, the source suggests.
The future diplomatic relations between the US and the Vatican depends in large part on the choice of ambassadors.
William Wilson, longtime friend of President Reagan and a convert to Catholicism, is the US representative to the Vatican. He is expected to continue as ambassador, as is the Vatican representative in Washington, Msgr. Pio Laghi.
Mr. Laghi is an experienced papal representative. He served as nuncio in Jerusalem, Cyprus, and, from 1974 to 1980, in Buenos Aires under the presidency of Isabel Peron and the subsequent military dictatorship of Gen. Jorge Videla.
In Washington, where he was moved in 1980, he mediated between the Vatican and US bishops over the American episcopal document on nuclear disarmament.
''There will probably be somewhat more Vatican influence on the proposed bishops' pastoral letter to be drawn up in November on capitalism and the economy than there would have been if diplomatic relations did not exist,'' says a seasoned left-wing Vatican commentator.