Defense Secretary Harold Brown said yesterday that the nation is over its Vietnam war hangover but cautioned against going on a new binge of interventionism.

The attitudes of the government and the public regarding a strong national defense "are very different than they were five years ago," Brown said at a Pentagon news conference.

The conviction is that "we need to be militarily strong." In fact, Brown added, "we need to be somewhat cautious to see that the pendulum doesn't swing back too far the other way to the point where we begin to believe that military strength can solve all of out international, let alone our domestic, problems."

The news conference followed another appearance by Brown before the Senate Armed Services Committee, where he again defended the administration's fiscal 1981 military budget which is 5.7 percent above what President Carter recommended for fiscal 1980, which began Oct. 1. The new budget includes money for a rapid deployment force to make it easier to respond quickly to trouble anywhere on the globe.

Even as Brown described the need for this quick-reaction force, he said that military strength alone does not equate with leverage in many situations.

The "preponderant influence" usually stems from "diplomatic, economic and political factors" rather than brute military force, he said.

However, he continued, "it's also true, and this is what we have come to realize, that in the absence of sufficient military strength" those other influences cannot be brought to bear in an effective way.

The quick-reaction force would be built from existing military units, with its first three brigades of Marines expected to be ready for action by 1983.

Brown said that about $10 billion will be spent over the next five years to buy large cargo planes and ships for the force. The ships would be stationed near likely trouble spots. Headquarters will be at MacDill Air Force base near Tampa, Fla., under the command of a three-star general.

Under current planning, part of the force could be assembled, equipped and deployed to the Persian Gulf area or elsewhere within seven days, according to Brown.

Questioned by Sen. John C. Culver (D-Yowa), Brown agreed that he saw no way such a force could rescue the American hostages in Iran. But he said a quick-reaction capability might be vital in the future.

Culver asked Brown to document the "threat differential" that prompted Carter to recommend a 5.7 percent increase.

The senator said he did not agree that defense spending should be increased in response to a "perception" that the United States is losing the military edge to the Soviets.

"Why does the American taxpayer have to spend billions and billions of dollars to cater to ignorance about the relative strength of the United States and Soviet Union?" Culver asked. "I'd rather hire a good ad firm in New York and let them buy time to explain what the real facts are."

Brown replied that "I have accurately represented the situation. The United States is not now inferior to the Soviet Union in overall military strength." But, he continued, future trends require higher spending.

Sen. Gary Hart (D-Colo.) came to Brown's defense as others on the committee suggested that the proposed increases were an attempt to sell the strategic arms limitation treaty to the Senate rather than respond to any new military threat."This committee is a little like a religious council saying, 'Get religion," Hart said, "and when you get religion, it says 'What changed your mind?'"

Brown said the word "innuendo" correctly described the criticism some committee members had leveled at the proposed increase.

In another exchange, Brown conceded to Sen. Carl Levin (D-Mich.) that he has outlined the administration's general thinking about future defense spending to former secretary of state Henry A. Kissinger. Kissinger has said he could support SALT II only if defense spending were increased.