A conservative committee that once counted Ronald Reagan among its members said yesterday that the so-called "window of vulnerability" to Russian missile attack that Reagan pledged to close actually "will open wider . . . over the next five years . . . due to the administration's decisions."

The assessment appears in a report by the Committee on the Present Danger that claims that the president's $1.6 trillion, five-year defense plan is still $100 billion too low and "will not halt the unfavorable trends in the U.S.-Soviet military balance, let alone reverse them."

While refraining from critizing the president directly, the report calls Reagan's defense program "minimal."

The committee, a bipartisan private organization of 200 conservative specialists in foreign and defense policy, was formed in 1976 because of a common concern that the United States faced a growing threat from Soviet military power.

The group has attracted considerable attention for articulating conservative views on this subject, and almost 40 of its members have landed positions in the Reagan administration, some at high levels. Reagan was a member until 1979 when, according to committee rules, he had to give up his membership upon announcing his candidacy.

The report issued yesterday is called "Is the Reagan Defense Program Adequate?" Its findings were presented at a press conference by Charls E. Walker and Herbert Stein, two economists who served in the Nixon administration and are now members of Reagan's economic policy advisory board, and William Van Cleave, a California professor and defense specialist.

Van Cleave headed Reagan's transition team at the Pentagon but was denied a spot in the new administration in part because he openly criticized Reagan's decision to deploy MX missiles in improved versions of old Titan missile silos.

Van Cleave's views appear to account for those areas of the report where the differences with the Reagan administration are sharpest.

The report says, for example, that U.S. strategic nuclear forces "will remain vulnerable for most of this decade" and chides the administration for a string of cutbacks in old missiles, bombers and submarines while failing to either decide on what to do with MX or to come up with a "quick fix" to improve the ability of current Minuteman missiles to survive a Soviet missile attack.

The report compares the administration defense program with recommendations the committee made in 1980 and finds the official levels of financing "clearly inadequate." The report calls for Reagan to move more rapidly toward the goal of spending 7 percent of gross national product on defense.

Although it estimates $100 billion more is needed, the report doesn't say specifically what it would be for, other than presumably an expansion of the Army and the strategic nuclear programs.

Although the majority view in Congress these days is to cut defense to help cut the huge federal deficit, Stein says that "it is ridiculous to say we cannot afford" what is necessary in defense. Stein, pointing out that 9-10 percent of GNP went toward defense in the Eisenhower and Kennedy administrations, claims that the American economy is not weak and thus the argument that a strong economy is needed before a strong defense is not valid.

Walker said that all the talk about whether the nation can afford the president's budget actually "distracts attention from whether what the president proposed is adequate or not."

Stein argued that there are ways to solve the deficit problem "which we think are infinitely preferable to cutting defense."

When asked how he would cut the deficit, Stein was reluctant to be specific but suggested "some change in entitlement programs" and "something" along the lines of an energy tax and a "good deal" that could be done with income taxes.