Secretary of State Cyrus R. Vance and other high officials began meetings yesterday with senior diplomats of the two giants of Asia, Japan and the People's Republic of China.

Chinese Deputy Foreign Minister Zhang Wenjin, highest ranking Chinese diplomat to come here since the visit of Vice Premier Deng Xiaoping 13 months ago, began his talks with an exchange of views about countering the Soviet invasion of Afghanistan.

Japanese Foreign Minister Saburo Okita, who is to arrive tomorrow for separate talks with U.S. officials, also is expected to focus on the changed international picture in the wake of the Soviet invasion.

State Department officials said the meetings with both Chinese and Japanese diplomats the same week it a conincidence, and is not meant to suggest a Washington-Peking-Tokyo alliance against the Soviet Union.

Officials said they consider it significant, however, that normalization of U.S. relations with China and the earlier normalization of Sino-Japanese relations make it possible for high-level talks to take place almost simultaneously here.

Washington's relations with the two countries are different, in keeping with different circumstances:

China, a communist state with about 1 billion people and a per-person income of about $500, is of major importance to the United States largely for strategic reasons. With more than 4 million men under arms, a small force of nuclear weapons and a long border with the Soviet Union, China is a power of much military importance but little economic importance now.

Japan, a democratic country with 115 million people and a per-person income of about $7,500, is a world economic power and one of the most important U.S. trading partners. U.S.-Japan trade last year was about $44 billion, about 20 times U.S.-China trade, bringing large economic benefits as well as much economic friction. Japan's military force is only 240,000 men, with no nuclear weapons or offensive capability.

Sino-Japanese relations are increasingly friendly, particularly after a promise by Tokyo late last year to provide capital for China's modernization. pBut despite Chinese overtures, Japan so far has been cautious about providing technology that could be applied to weapons. One reason is to avoid friction with the Soviets.

The Chinese diplomat's talks this week are expected to center on "parallel" steps that Washington and Peking can take to strengthen Pakistan and oppose Soviet expansion in Southwest Asia. China long has been Pakistan's closest ally. Peking is reported to have aided some factions of the Islamic insurgents in Afghanistan for many months.

In advance of Zhang's visit, there was no indication China is interested in joint Sino-American measures to aid Pakistan as suggested by Secretary of Defense Harold Brown in Peking in January. China was reported wary of appearing too closely allied to the United States in the eyes of developing nations. Pakistan's quarrel with the United States over military aid was anther blow to a joint Peking-Washington effort.

China also has made clear it is not interested in a Washington-Peking "hot line" -- similar to the Washington-Moscow link -- for quick communication in time of crisis, according to U.S. sources. This was also suggested by Brown in January.

The United States announced after Brown's trip that it will supply selected "dual use" technology of military value and some nonlethal military equipment to China. A munitions control letter permitting sales of these items, including defensive radar, has been approved by the State Department for publication within a few days, officials said.

The Japanese-American talks are expected to dwell more than in the past on the world-wide strategic situation, although international and bilateral economic issues are expected to receive major emphasis.

A public dispute in December was touched off by large Japanese purchases of Iranian oil while the United States was seeking to place economic pressure on Iran because of the hostage crisis. Officials on both sides said the mutual irritation has been surmounted, although, it could recur if Washington swings back to a policy of economic sanctions against Iran.

Japanese contributions to the costs of U.S. military forces in Japan and tension over Japanese exports of automobiles and other items here are also on the agenda.