Secretary of State Cyrus R. Vance was cautioned today against raising unrealistic expectations for an early settlement of the Arab-Israeli dispute. If soaring expectations are not met, he was told by King Hussein of Jordan, the disillusionment could be "unhelpful and dangerous."

Hussein's warning was made known in a session with reporters under "background' rules of attribution by a senior U.S. official as Vance's plane flew toward conferences here with leaders of Saudi Arabia. The identity of the official cannot be reported under the [WORD ILLEGIBLE] ,but his duties are just about the same as those of the famous eled abroad in Henry A. Kissinger's party.

Early in the new administration, Vance made it known tha the "senior official" had been retired from service in favor of on-the-record briefings. Press statements for direct quotation from a Secretary of State, however, can cause a diplomatic tempest if not phrased with extraordinary care.

Despite reporters' inquiries, the U.S. official declined to comment on the substance or the impact of The Washington Post report that Hussein had received secret CIA payments over 20 years. This evidently reflects a policy decision that the less said about it, the better.

In a departure statement at Amman before television cameras and reporters, Vance announced that Hussein has accepted an invitation to visit Washington inApril. He went out of his way to praise Hussein's "important and moderating influence" on the turbulent Middle East and to reaffirm the "close and harmonious relationship that exists between our two countries." Vance added, "I have stated unequivocally [to Hussein] the commitment of the United States to Jordan's economic progress and to our cooperation in the pursuit of peace." He diclined to take questions from the press.

There was no way to assess at this stage the damage which the Post revelations may have on Hussein's standing within Jordan and the Arab world. Jordanian newspapers carried the official condemnation of the story this morning without giving details of the Post account.

Reports on the Post story were being broadcast to Jordan via Israeli state radio, however, and the nature of the Post account was becoming more widely known to the Jordanian citizenry.

As ruler of a royal buffer state between far more powerful nations in this explosive region, Hussein has been in a delicate position for many years - particularly after he ousted Palestinian revolutionaries from his soil in a 1970 civil war.This precarious situation is the wellspring of the concern expressed to Vance about overopitimism regarding Middle East peace. With a population composed predominately of Palestinian Arabs, Jordan could be thrown into an upheaval by the rise of passions over unfulfilled hopes for peace.

About one-third of Jordan's population of 2.3 million lives under Israeli occupation on the west side of the Jordan River. This gives Hussein's kingdom a massive stake in an Arab-Israeli peace settlement involving Israeli release of territory occupied in the 1967 war, either by returning it to Hussein's rule or establishing the now-occupied lands as a new Palestinian ministate.

Vance and Hussein conferred at length about Egyptian President Anwar Sadat's proposal for a conferderation to be negotiated between Hussein and Palestine Liberation Organization authorities who could be leaders of a future West Bank ministate.

Hussein, who is scheduled to meet with a delegation of PLO leaders next week, does not rule out such a confederation but feels that other plans for future control of the West Bank area must also be considered, Vance was told.

The senior official on Vance's plane said Hussein has kept in touch with the thinking of the PLO despite the bitter attacks leveled on him by the Palestinian revolutionaries following their ouster from Jordan. The officials said almost all of the leaders Vance has seen are in close touch with the PLO, which the United States does not recognize or deal with on an official level.

PLO chairman Yasser Arafat, who was in Cairo just before Vance's arrival there earlier this week, was conferring with Saudi officials in their capital at Riyadh just before Vance's arrival here today. The senior official said he wondered but did not know what Arafat has been doing in the round of conferences with Arab leaders.

PLO representatives have told reporters that Arafat's current travels are related to Vance's mission, giving rise to speculation that Arafat is trying to influence the positions of Arab leaders about Palestinian questions in their talks with Vance. Arafat left for Kuwait as Vance arrived.

According to the senior official, Lebanese President Elias Sarkis raised with Vance yesterday, the possibility that Lebanon might be included as a party in a new Geneva conference on Middle East peace. Vance told Sarkis the United States sees no reason why this should not be done, but pointed out that invitations to a peace conference must be approved by other nations concerned.

Lebanon was not among the parties to the 1973 Geneva conference because at the time it was not considered a state in confrontation with Israel. The civil war that flared last year with Israel brought large numbers of Syrian troops into Lebanon, increasing the potential for a clash with the Israelis.

Vance began his talks here this afternoon with Saudi leaders in the richly decorated palaces that are headquarters for this oil-rich state. The talks, going into the night at a royal dinner and possibly after-dinner conferences, were expected to touch on international economic conditions brought about by the sarcity and price of oil as well as Saudi views on Arab-Israeli peace.

News agencies reported these and other Middle East developments:

Lebanese Christian forces overran the Moslem town of Khiam two miles north of the border with Israel. The town of 10,000 once served as a base for Palestinian attacks into Israel. A refugee arriving in Beirut said 21 persons were killed in the fighting yesterday. Others put the toll at 50. Israeli residents near the Lebanese border reported today that there were sounds of heavy fighting in Lebanon and that Christain forces said their drive was continuing.

The Soviet Union accused Egyptian President Sadat of writing "lies, slander, and falsification" about Soviet-Egyptian relations in his political memoirs now running in a Cairo weekly.

The unusually vitriolic editorial in the Communist Party newspaper Pravad seemed certain to heighten strains in Egyptian-Soviet ties, which have grown steadily worse over the past year.

The Egyptian state security court ordered the release of 70 people detained after last month's price riots and accused of connections with clandestine Communist organizations. The court denied bail requests from 37 others.