Shah Mohammad Reza Pahlavi will leave Iran on vacation for an indefinite period after naming a regency council "in the next few days" to rule in his place, Secretary of State Cyrus R. Vance said yesterday. Vance called the move "a sound decision."

Speaking at a press conference, Vance established officially for the first time the U.S. expectation of an imminent departure by the shah. The secretary's description was based on statements the Iranian ruler has made directly to U.S. officials, State Department sources said later.

In other comments, Vance in effect ruled out the possibility that the Carter administration would build a "falling domino" theory for South Asia and the Middle East out of the turmoil in Iran and what some other officials and commentators see as Soviet gains in Afhganistan, Pakistan, the Horn of Africa and elsewhere.

"I think it does not help to oversimplify the problem," Vance siad in response to a question. "We have different countries here and although you have common factors which affect these various countries, that does not mean that the problem is identical in each."

With his answer, Vance put more clearly into public view than he has before his continuing philosphical differences with President Carter's national security affairs adviser, Zbigniew Brzezinski, who has recently stressed in public comments that Iran is part of "an arc of crisis" open to foreign subversion.

National Security Council spokesman Jerrold Schecter strongly decied that there were any differences between the views of Vance and those of Brzezinski. But senior aides to Vance are increasingly concerned about what they call "the single answer" approach of "standing up to the Russians" in dealing with Third World problems.

Vance yesterday offered the administration's most muted support for the shah yet put on the record. Instead, he highlighted the administration's support for the efforts of Prime Minister Shahpour Bakhtiar to form a civilian cabinet and urged the Iranian military to support a civilian Iranian regime.

Concern continued to deepen within the administration, however, over whether Bakhtiar's cabinet would win the support from the shah's political opponents and from the Iranian armed forces it needs to survive. Vance's timetable for the shah's departure seemed to take into account the possibility that it would be put off if a civilian government is not firmly installed first.

The administration has under consideration plans to expand and arm more heavily U.S guard units around intelligence-gathering installations in Iran and has begun dismantling some of the highly sensitive equipment there for possible emergency evacuation, U.S. sources said yesterday.

Vance's press conference, which was dominated by Iran, was in itself a sign that the State Department has taken the lead in dealing with that country as the shah's chances to regain full power have evaporated in the heat of social protest against his rule.President Carter canceled a press conference this week where he would have had to speak on Iran.

Brzezinski appeared to dominate U.S. policymaking on Iran in the autumn, when the administration constantly reiterated its full support for whatever decisions the shah felt necessary to handle the crisis.

Reading a seven-paragraph statement yesterday, Vance did not cite the shah's reocrd as a long-time U.S. ally. Instead, he emphasized that the administration was dealing with the shah as a "constitutional head of state."

U.S. officials declined to say if the shah has advised the administration whom he will name as regent, or where he will go on a trip from which he may never return. American and foreign diplomats list the United States and Switzerland as the two most likely destinations.

Vance's rejection of a "falling domino" theory for Iran similar to the one long used to justify U.S. policy in Southeast Asia was intended in part to rebut former secretary of state Henry A. Kissinger's assertions that the Carter administration is responding weakly to Soviet intrustions in Africa, Asia and the Middle East, Vance aides said.

Asked about turmoil in Iran and its neighbors, Vance replied:

"Fundamental changes are taking place in the area in terms of economic modernization, social changes and demands for wider political involvement."

Saying that the United States wanted to remain close to friendly countries in that area, he cautioned against falling "into the trap of accepting oversimplified generalities as applying to differing situations" and said U.S. policy "must be applied with care and precision."

Acknowledging that the U.S. recognition of China and American involvement in sorting out the Iranian crisis have made the Soviet Union more jittery than usual about U.S. intentions, senior State Department aides suggested that Vance's analysis and lowkey responses on questions about Soviet actions were intended to put Brzezinski's more starkly worded concerns into a different perspective.

Vance said that there was no clear evidence that the Soviet Union had played a role in the Vietnamese invasion last week of Cambodia and noted that much of the equipment used in the invasion was captured American materiel. To another question, he said "there will be no tilts one way or the other" in U.S. relations with China and the Soviet Union.

While White House aides said privately that Brzezinski sees his role as one of giving more of a global overview of international problems, NSC spokesman Schecter insisted that any suggestion that Brzezinski's "are of crisis" analysis "is any different than that of Secretary Vance's views is merely artificially manufacturing differences" between the two officials.