Vietnam's ruling Communists concluded a party congress yesterday by dropping six members from the powerful Politburo, including the country's best-known figure, Vo Nguyen Giap.

Giap is credited with devising the military strategy that defeated France's attempt to retain its possessions in Southeast Asia after World War II.

The shake-up in the Politburo was matched by similar reshuffling of the Central Committee and that body's secretariat. According to reports carried by Vietnam's official news media, among those losing party positions were such veterans as Xuan Thuy, who led the North Vietnamese delegation to the Paris peace talks in 1968, and Nguyen Duy Trinh, a former foreign minister.

The scope of top leadership changes is without precedent for the Vietnamese Communist Party. The party has long been notable among Marxist nations for its stability.

There seem to be several factors behind the purge. Age and poor health were apparently the reasons some lost their party posts. For others, such as Le Thanh Ngi, who was dropped from the Politburo, it apparently was association with the unsuccessful economic planning of the past few years.

Another factor seems to be an intent to reduce the number of people holding both party and government positions. In a speech last Saturday to the congress' opening session, Le Duc Tho, who retained his place on the Politburo, spoke out against dual responsibilities. Too many Polituro members, Tho said, were vice-premiers in the government and were unable to devote themselves fully to party affairs.

Giap, who is 70 this year, has reportedly suffered from periodic bouts of poor health. In 1980 he lost his post as defense minister but was named deputy premier in charge of science and technology.

In a report from Hanoi yesterday, Agence France-Presse quoted Huang Tung, a director of the party newspaper, Nhan Dan, as saying that Giap decided, "along with others, not to be a candidate," in the Politburo elections. Giap retained a position on the party's Central Committee.

Nothing in the changes announced yesterday indicates that the Vietnamese contemplate any change in foreign policy, least of all in the occupation of Cambodia.

Succeeding to Giap's ranking as sixth in the party hierarchy--as he did earlier to the post of defense minister--was Gen. Van Tien Dung, who was field commander of the 1975 offensive that eventually toppled the U.S.-backed government in South Vietnam. More recently, Dung planned the Vietnamese drive into Cambodia in 1978.

One of the new members of the Politburo is the current commander in Cambodia, Lt. Gen. Le Duc Anh. Named alternate member was Don Si Nguyen, a general responsible for the logistics of the invasion.