A presidential commission yesterday proposed deployment of the MX intercontinental ballistic missile in existing Minuteman silos in Wyoming and Nebraska, and concluded that the "window of vulnerability" that President Reagan made a major issue in the 1980 campaign is not as serious as he contended.

The report of the President's Commission on Strategic Forces, a blue-ribbon group including six former defense secretaries among its 11 members and eight advisers, called for development of a much smaller, mobile missile with a single nuclear warhead that it said would make a less inviting target for the Soviets than the mammoth MX, with its 10 nuclear warheads.

The panel also proposed counting warheads rather than launchers in arms control negotiations with the Soviet Union. This ultimately might require revision of the Reagan administration's proposal at Geneva to limit long-range U.S. and Soviet nuclear weapons.

The new proposal, which drew heavily on the expertise of Carter administration defense experts, amounts to a bipartisan political compromise to salvage the MX after two Reagan administration proposals were rejected by Congress.

Commission Chairman Brent Scowcroft, a retired Air Force general and national security adviser in the Ford administration, said the report represented a "consensus" approach that had "the best chance" of success on Capitol Hill.

However, the Senate rejected an earlier plan for interim basing of the MX in Minuteman silos, partly on the grounds of their supposed vulnerability. The House in December turned down another administration plan calling for clustering the missiles in a system known as "Dense Pack."

The commission estimated the five-year cost of deployment of the 100 MX missiles in Minuteman silos and the future development--but not the deployment--of smaller missiles at $19.9 billion, compared with $27.9 billion for the Dense Pack plan. Reagan has offered this savings to Congress as a possible reduction in the cost of his defense buildup.

Thomas C. Reed, the commission vice chairman and secretary of the Air Force under President Ford, said the difference between the new proposal and the previous two was that this time Congress was consulted on what would be acceptable in an attempt to build a long-term political coalition behind the MX.

"It isn't like passing a law and then challenging your successors to repeal it," Reed said. "A weapons system is in the interests of the whole nation. It requires an orderly research, development, production and deployment timetable that takes several political generations to complete."

At the request of House Republican leaders who thought that presidential backing for the MX and the new missile might complicate efforts to defeat the nuclear freeze resolution on Wednesday, Reagan put off his formal endorsement of the plan until next week. But, during a photo session with commission members yesterday morning, he joked that "some of my best friends are MX missiles."

As a practical matter, White House officials acknowledged, Reagan has little choice except to accept the commission's basic proposals because to do otherwise would upset the delicate framework for bipartisan cooperation that the report has sought to create. Officials said he would embrace the report formally in a statement next Tuesday. Congress will then have 45 days to act on the recommendations.

The administration is heavily dependent on the support of Harold Brown, secretary of defense in the Carter administration, and William J. Perry, his scientific deputy. Brown, a counselor to the commission, played a leading role in its news conference yesterday and issued a statement differing with the Reagan administration on some points but calling the commission's recommendations "a reasonable program, and on balance the best available one, for the modernization of the U.S. ICBM force."

"The United States needs to promote deterrence by demonstrating to the Soviet Union and our allies that we will maintain a modernized strategic nuclear force essentially equivalent to that of the Soviet Union," Brown said.

One of the paradoxes of the administration's current reliance on the support of its predecessors is that Reagan, as a presidential candidate in 1980, repeatedly contended that a large "window of vulnerability" existed in relation to the Soviets. Reagan also scoffed at Carter's plan to scatter 200 MX missiles amid 4,600 shelters in the Utah and Nevada deserts.

The commission report had kind words for the Carter plan, but took note that "local political opposition to it has been significant."

The report also acknowledged the vulnerability of the MX in Minuteman silos, but minimized the importance of this on grounds that the Soviets could not effectively attack U.S. missiles and submarine and bomber bases at the same time. This in effect contradicted the Reagan contention that the United States is imperiled because of a growing "window of vulnerability" involving strategic nuclear weapons.

" . . . The different components of our strategic forces would force the Soviets, if they were to contemplate an all-out attack, to make choices which would lead them to reduce significantly their effectiveness against one component in order to attack another," the report said.

"For example, if Soviet war planners should decide to attack our bomber and submarine bases and our ICBM silos with simultaneous detonations . . . , then a very high proportion of our alert bombers would have escaped before their bases were struck."

The commission report argued that it was the vulnerability of the entire system that was important and said that "different components of our strategic forces should be assessed collectively and not in isolation."

This was an argument that was unpersuasive to candidate Reagan, as Brown suggested at the commission's news conference yesterday.

"If one wants to go back to the 1980 campaign, perhaps some of you who were involved in reporting it will remember what was said by one side was that there was a critical window of vulnerability and that it needed to be removed right away," Brown said. "And what the other side was saying is, 'This vulnerability is a problem. We are going to have to take care of it.' And the timing and the overall nature of the problem was not that immediate and that critical."

A key recommendation of the report was for development of a small missile that would weigh 15 tons, about one-seventh the weight of the MX, to be deployed in the early 1990s. The commission envisioned that it could be used to encourage the Soviets to "move toward a more stable ICBM force" so that the United States would not have to deploy large numbers of the new missiles.

"We should keep in mind, however, that having several different modes of deployment may serve our objective of stability," the report said. "The objective for the United States should be to have an overall program that will so confound, complicate and frustrate the efforts of Soviet strategic war planners that, even in moments of stress, they could not believe they could attack our ICBM forces effectively."

The commission report strongly reaffirmed the basic U.S. strategic concept of the "triad," which holds that a mix of airborne, seaborne and land-based missiles should be maintained as a deterrent to the Soviets.

The report called for continued deployment of the Trident II missile-firing submarine and for research on developing a smaller submarine that would present a more difficult target.

The commission made no recommendations for change in the Reagan administration's support of the B1 bomber. It called for "vigorous research and development" of anti-ballistic missile defenses but said existing technologies do not justify proceeding beyond this point. In making its report yesterday, the commission abandoned the argument of some administration officials that the MX should be deployed because it is "a bargaining chip" in arms control negotiations.

"This is not a bargaining chip," Reed said. "What is a bargaining chip is what we will be doing if the Soviets don't come to the bargaining table," a reference to the potential for deploying additional hundreds of MX missiles.

The report marked the end of Reed's tenure as an administration consultant. Because of his role in a stock transaction that is under investigation, Reed announced last month that he was leaving his consultantship effective with the submission of the report.