Former secretary of state Cyrus R. Vance criticized the Reagan administration in uncharacteristically blunt terms yesterday for failing to manage, or even understand, major world issues, but at the same time he welcomed the administration's new strategic arms proposal as "a major step forward."

Vance, who is usually sparing in public criticisms, condemned current policies, and the absence of policies, across a broad front, saying, "There is no sense that the Reagan administration has a basic understanding of the forces that are shaping today's world, much less those that will shape tomorrow's."

He also charged that the administration had "needlessly compounded difficulties" within the NATO alliance, "let slip the reins of effective policy" in the Middle East, ignored the impact of domestic economic policy on other nations, eroded U.S. relations with developing countries and "dimmed the beacon hope" of human rights for millions of people facing political oppression.

Vance gave his views in a paper written for the Center for National Policy, a think tank aligned with the Democratic Party. He discussed the paper at a National Theater program sponsored by the Foreign Policy Association and World Affairs Council, and in a meeting with reporters.

The proposal for extensive cuts in strategic nuclear arms announced by President Reagan Sunday drew commendation from Vance, who called for "a balanced approach" to U.S.-Soviet relations rather than an approach based on harsh rhetoric.

It was Vance who in March, 1977, took President Carter's "deep cuts" nuclear arms proposal to Moscow, only to meet an immediate rejection from the Soviets. It took many months to put the strategic arms negotiations back on track.

Vance said Soviet officials have since told him that they made a mistake in dismissing this plan out of hand. He predicted that the Soviets, therefore, will be willing to consider Reagan's plan as "an opening position of the United States."

In order to make progress within the time available to this administration, he added, it will be necessary to have "a very clear affirmation" of the essential terms of the SALT II treaty, which Vance negotiated and Carter signed, but which was opposed by Reagan.

Vance said he believes it is possible that the Soviet Union would agree to the reservations attached to the treaty in the Senate Foreign Relations Committee.

Conceding that it is "a hard row to hoe" to convince the administration to accept SALT II, even in modified form, Vance argued that, without that foundation with the Soviets, "you'll be back to square one" in nuclear arms negotiations.

Without endorsing it as an objective of policy, Vance had words of encouragement for the drive for a U.S.-Soviet nuclear weapons freeze. This has "a really important political function" in educating the public and pressuring the administration to resume arms control efforts, he said.