Iranian President Abol Hassan Bani-Sadr today gave vocal approval for an international commission to go to Iran and begin an inquiry into the alleged crimes of the deposed shah, Mohammad Reza Pahlavi. The move was seen as a key step toward eventual release of the American hostages held in Tehran.

Although Bani-Sadr failed to set a time for freeing the Americans, diplomatic sources said that he is believed to have assured U.N. Secretary General Kurt Waldheim today that Ayatollah Ruhollah Khomeini, who controls the militant captors, would publicly support the release after the commission arrived in Tehran, which would probably be by Thursday.

U.N. mediators continued to withhold a public unveiling of the panel until receiving written confirmation of Tehran's approval.

[The Reuter news agency said the embassy militants in the Iranian capital held an emergency meeting today with Kohomeini's son, Ahmad, but that the captors said they were ordered not to disclose details of the talks. However, they said they planned to issue a statement "shortly" on the five-man commission, according to Reuter.]

Bani-sadr's approval of the commission came in an early morning telephone call to Waldheim. It is said to have inched the mediation effort closer to an agreement to release the estimated 50 hostages held at the U.S. Embassy in Tehran for 15 weeks. Three other Americans are being held at the Iranian Foreign Ministry.

With Bani-Sadr's written approval expected by Tuesday, the cochairmen of the five-member panel -- Andres Aguilar of Venezuela and Mohammed Bedjaoui of Algeria -- conferred with Waldheim in prepraration for their trip to Geneva tonight or Tuesday. They are scheduled to leave for Tehran Wednesday with other commission members.

U.S. Secretary of State Cyrus Vance also met with the two men and Waldheim in separate meetings at the world organization. Vance left the U.S. chief's office suite after seven hours, refusing to answer substantive questions and refering reporters to the secretary general.

Characterizing the talks as "useful," Vance said he reviewed the situation "in depth" with Waldheim and planned to return immediately to Washington to report his discussions to President Carter.

Vance met with the U.N. secretary general on the eve of his departure Tuesday on a European tour that will take him to West Germany, Italy, France and England.

The tour was described as part of continuing consultations with allies to coordinate the Western response to the Soviet intervention in Afghanistan.

Waldheim's mediation efforts included talks not only with Vance but also with the charge d'affaires of the Iranian U.N. mission, Jamal Shemirani.

U.N. spokesman said Waldheim was merely following routine diplomatic practice in waiting for written confirmation from Bani-Sadr before naming the panel, which is considered the first step in the complex process leading to a hostage release.

But informed sources indicated that the Iranian president may be holding out for more time to guaranteee cooperation from Khomeni, the ailing revolutionary leader whose support is considered vital in extricating the militants from the embassy without force.

Khomeini has remained silent on the mediation effort, although Bani-Sadr claimed that he has the religious leader's backing and is believed to have privately assured Waldheim that Khomeini would publicly support the initiative after the commission arrives in Tehran.

After Waldheim unveils the commission members and states the panel's mandate, governments in Washington and Tehran are expected to issue separate statements giving their views of the fact finding mission. Sources said Khomeini would address his nation sometime later, possibly on the Friday prayer day, a day after the commission arrives in the Iranian capital city.

The commission's travel plans appear to reflect a change in U.S. policy. In recent days U.S. officials have insisted that Bani-Sadr set a fixed time for the hostage release before the panel could fly to Tehran and begin its investigation.

Diplomatic observers believed that the United States has now agreed that the commission should be allowed to go to Tehran, with the understanding that Khomeini would make a public statement of support for the hostages' release after its arrival, thus setting in motion the process leading to their freedom.

U.S. officials refused to discuss any facet of the mediation effort. But the panel's preparation for Tehran suggests to the observers that U.S. officials are now willing to give Bani-Sadr more time to arrange a release date.

As a condition of his public support, Khomeini is said to have demanded that the commission land on Iranian soil and demonstrate serious intentions to inquire into the nation's grievances against the shah, who now lives in Panama.

Iranian authorities supervised by a third party would take control of the embassy from the militant youths, who have refused to release the Americans until they either are ordered to do so by Khomeini or the deposed shah and his fortune are returned to Iran.

It is still unclear when Khomini would publicly place his imprimatur on the Waldheim plan, although sources said it would probably come before the commission begins "effective work," possibly a few days after the panel arrives in Tehran.

Current plans for the panel call for a two-week stay in Tehran where members would collect information on Iranian complaints and hear witnesses. Complying with U.S. demands, the group would limit its scope to fact-finding and avoid moral judgements. A final report would be submitted to Waldheim.