The Soviet Union today named an aging diplomat and longtime Communist Party official to the newly created post of first vice president, thus delaying any public decision on the possible succession to Leonid I. Brezhnev, the 70-year-old chief of state.

By unanimous vote, the Supreme Soviet named Vasili V. Kuznetsov, 76, a foreign affairs specialist, to the new job, passing over much younger men of the Politburo who have built and maintained strong political bases in such sensitive regions of the country as the Ukraine and Leningrad.

In the view of some informed Western observers, Kuznetsov was chosen because, in the words of one. "It is simply too difficult for the Kremlin to settle a succession while the leader is still in power. It opens up too many rivalries ahead of time."

Kuznetsov has been first deputy foreign minister for the past 22 years. A metallurgist by training, he is a rarity in the Soviet leadership in having earned a master's degree in the United States - at Carnegie Institute of Technology in Pittsburg. He worked in a Ford auto plant in 1931 under an exchange program. He speaks fluent English and is considered by U.S. authorities to be an expert on the United States and United Nations.

The legislative action on Kuznetsov was simply a rubber stamp of the Politburo's choice. The 14-member inner-most ruling circle of the Communist Party offered Kuznetsov as the only candidate.

The Supreme Soviet, the parliament, has been meeting in a special four day session to adopt a new constitution to replace that of 1936. The new constitution was adopted unanimously today and put into effect immediately. The naming of Kuznetsov to the new vice presidency came minutes later.

Foreign diplomats and some Soviet sources had suggested earlier that the creation of the new vice presidency in a constitution whose writing was under Brezhnev's direct control was an opportunity for him and his colleagues to establish a possibly orderly line of succession. Brezhnev has been in uncertain health.

Earlier this year, Nikolai Podgorny was dismissed as president and Brezhnev took his position. This made one man both party leader and head of state, giving Brezhnev more authority than any other Kremlin leader since Stalin, who died in 1953.

The Politburo is a gerontocracy - five of 14 members are 70 or more, three others are at least 65, three are in their early 60s and three are in their late 50s.

Among the younger men are some who have been clearly moving forward in the secretive inner leadership. These include Yuri Andropov, 62, the head of the secret police; Vladimir Scherbitsky, Ukrainian party chief who recently received his second Hero of Socialist Labor award; G. V. Romanov, tough head of the Leningrad region, and F. D. Kulakov, the minister of agriculture who has been blessed with two successive excellent harvests.

Kulahov sat in the front rank of Politburo members with Brezhnev and Soviet Premier Alexei Kosygin during this week's Supreme Soviet sessions.

This summer, French diplomatic sources who had studied Brezhnev closely during his state visit there reported that he was in seriously failing health, lacking in stamina and at times seemingly in a daze. He has had various undisclosed ailments for some years. This weeks, he has appeared in command of his faculties during his two speeches to the legislature.

Today, in two incidents, Brezhnev was seen to be holding his hand up to cast a vote long after the vote had been taken. In one instance, party theoretician Mikhail Suslov reached over and clearly told him to bring his hand down. Brezhnev, like other Politburo members a man with a remarkably impassive countenance, broke into a grin and dropped his hand.

The new constitution adopted today - the Soviet Union's fourth since the 1917 revolution - proclaims the nation the world's first "developed socialist state," gives a detailed definition of a citizen's rights in relation to his duties, and seeks to end painful memories of Stalinist purges 40 years ago.

At the same time, the new document enshrines the Communist Party as the leading force in the state, the moral arbiter of the societ. Enactment of the new document marked a moment of high trumph for Brezhnev, party leader since the ouster of Nikita S. Khrushchev 13 years ago.

The new constitution is Brezhnev's brainchild. He has overseen work on it since 1962, when Khrushchev sought to revise the 1936 "Stalin constitution" linked in the minds of millions of Soviets with the purges of the late 1930s, in which unknown thousands were imprisoned and executed.

At 9,000 words, the Brezhnev constitution is approximately three times the length of the 1936 document.

In a prepared speech to the Supreme Soviet after their quick vote - 13 minutes to approve; all ayes, no nays, no abstentions - Brezhnev declared: "The fundamental law of the world's first socialist state of the whole people has been approved . . . Years and decades will pass, but this day will always remain in the people's memory as vivid evidence of the genuine triumph of the Leninist principles of the people's power.

Brezhnev continued to show sensitivity to some Western authorities' criticisms that the new document has little real meaning in terms of changing the nature of Soviet society. "We have not created the constitution as a stage prop," he asserted. "It has to be fulfilled and will be fulfilled in all its parts."

First promulgated in draft form this past summer, it contains a preamble, nine parts and more than 173 articles. It sets forth the basic principles of the societ in a way that is similar to the U.S. constitution. While the Soviet constitution contains sweeping guarantees of personal rights and freedoms, however, it sets them within a framework of duty to the will of the state.

In section two, "The State and the Individual," the constitution sets forth detailed individual rights, from privacy through freedom of worship, speech, press and peaceable assembly, to such specific rights as to hold a job, have a pension, free medical care, a home, and education. But article 59 ties these freedoms directly to duty to the state:

"Exercise of rights and freedoms shall be inseparable from the performance by citizens of their duties." Such duties include "to work conscientiously in his chosen socially useful occupation and strictly to observe labor and production discipline," to care for one's family, to protect socialist property Article 62 asserts: "The citizen of the U.S.S.R. shall be obliged to safeguard the interests of the Soviet state, to contribute to the strengthening of its might and prestige."

The supreme role of the Communist Party is spelled out in Article six, which declares: "The Communist Party of Soviet Union is the leading and guiding force of Soviet society and all the nucleus of its political system, of all state and public organizations. The (party) exists for the people and serves the people."