Vietnam convened a rare congress of its Communist Party in Hanoi today amid indications of a reshuffle of the aging leadership, a broadened party purge and ratification of the country's new, more liberal economic policies.

According to Western diplomats here, there have been indications from Hanoi in the weeks preceding the congress--only the fifth since the party was founded in 1930--that Le Duan, 74, would retire as party general secretary, a post he has held for 22 years.

Most frequently mentioned as his successor has been Le Duc Tho, 72, the former chief negotiator at the Paris peace talks and political architect of the Communist takeover of South Vietnam. There has been speculation that Le Duan would move up to the more honorific post of party chairman, which has been vacant since the death of Ho Chi Minh in 1969.

According to Western diplomats here, any leadership changes are unlikely to affect Vietnam's foreign policy, on which there is no significant debate within the Communist Party.

In an address to the congress broadcast by Radio Hanoi today, Le Duan admitted to many errors and economic shortcomings in the country. He called for a major purge of the party membership after the congress ends in about a week.

Radio Hanoi said the congress would set economic policy guidelines for the 1980s and elect a new central committee.

"At present our country is faced with many pressing problems on the economic front," Le Duan said. "Production has developed slowly while the population has increased at a fast rate." He added that "grain, cloth and other essential consumer goods are all in short supply."

Le Duan blamed "many shortcomings and errors in leadership and management" that he said "constitute the main cause that has either triggered or further aggravated these economic and social difficulties in the past years."

He denounced some party members for failing to "uphold revolutionary quality," adding: "In order to keep the party ranks clean, following this congress we must resolutely expel from the party as soon as possible all opportunists, all those whose revolutionary spirit has been paralyzed, exploiters, smugglers, speculators, persons involved in corruption and bribery and oppressors of the masses."

The purge of "bad elements" from the party has been among the contentious issues that have cropped up in meetings preceding the congress, according to diplomatic reports. Tens of thousands of the party's 1.5 million members already have been dropped, and there has been intense debate about whether to halt or escalate the purge. The congress originally had been scheduled for December, but debates on domestic issues delayed it, diplomats said.

According to diplomatic reports from Hanoi, a new economic policy already has been hammered out in a series of unusually polemical preparatory meetings. The Vietnamese press has used such terms as "heated," "intensive" and "frank" to describe debates, which have pitted advocates of a more pragmatic, even quasi-capitalistic approach to the economy against purists wedded to orthodox communist economics.

The pragmatists succeeded in introducing a "new economic theory" in the wake of Vietnam's dismal performance under the 1976-1980 five-year plan, and this approach is expected to be upheld at the party congress. The main features of the new economics have been material incentives for workers and peasants and introduction of a limited free-market approach to increase food production. Production of consumer goods also has been given greater attention.

One particularly controversial feature has been the adoption of a "contract farming system" in North Vietnam allowing families on collective farms to sell produce exceeding their contract quota to the state or on the free market. The measure is believed to have contributed to Vietnam's production of a record 15 million tons of food grains last year--enough for basic subsistence but a million tons less than the level for self-sufficiency.

On foreign policy, Le Duan's speech strongly reaffirmed Vietnam's close links with the Soviet Union and its commitment to the Phnom Penh government in Cambodia, which Hanoi's troops installed in 1979 after their invasion. He also expressed firm support for Poland's martial-law government and the Soviet-installed leaders of Afghanistan.

Le Duan bitterly denounced China, telling the congress that "the most dangerous enemy of the nation is Chinese expansionism, in collusion with U.S. imperialism."