A wooden fishing boat full of Vietnamese refugees who were twice denied permision to come ashore hit a sandbar and sank here today and about 200 unwanted "boat people" apparently drowned in the rough waters of the Trengganu River estuary.

The disaster underscored the plight of thousands of Vietnamese who have been fleeing their country since the Communist government took over in 1975. Often they leave in small, over-crowded crafts, in search of a haven in nearby countries which proved unwilling to take them in.

Malaysia is refusing to accept any more of the refugees. About 35,000 already have found temporary shelters in this nation 300 miles southwest of Indochina. Home Affairs Minister Ghazali Shafie said the government regards them as illegal immigrants because if they were officially recognized as refugees, they would have a claim to stay permanently.

More than 2,500 refugees from Vietnam, most of them believed to be ethnic Chinese who paid in gold for authorization to leave, have been stranded for two weeks aboard a rusty freighter off the Malaysian city of Port Kland - refused entry into Malaysia but with nowhere else to go.

The United States, as part of an international effort to ease their travail, has offered to admit 2,500 more Vietnamese refugees from Malaysia's camps on a one-time basis, and the Carter administration is reported considering a proposal to double the annual quota of 25,000 for Indochinese immigrants into the United States.

Canada, France, West Germany, Belgium and Switzerland also have agreed to accept some refugees. But as Western officials screen applicants, refugees continue to arrive on Malaysia's eastern shore, many in small boats dangerously overfilled.

Malaysian authorities say that when the boats are sighted off the coast, police order them to move on if the boat is seaworthy.

The 60-foot boat that sank today, which survivors said carried 245 refugees including about 60 women and 50 children, first put in at the Malaysian island of Pulau Bidong about 25 miles north of here. It was turned away by police - who apparently judged the craft seaworthy - and showed up yesterday at the fishing village of Sebarang Takir at the mouth of the Trengganu River.

Refused permission to land, the Vietnamese insisted on staying, but hundreds of villagers gathered around the boat and pushed it back out into the river, the villagers said.

The chief minister of Trengganu state, Wan Mokhtar, said police then asked the Vietnamese to follow another boat taking supplies to Pulau Bidong but the refugees craft took the wrong channel and rammed the sandbar in swift currents leading into the South China Sea.

Rescue boats were not sent out because of the currents and rough seas, police said. Villagers threw inflated inner tubes into the water for swimming refugees to grab, but authorities said only 51 survived.

They will be taken to Pulau Bidong, where they were turned away Tuesday, and installed in a U.N.-financed camp that houses more than 20,000 refugees.

Home Affairs Minister Shafie said the government set up a task force today to slow the flow of refugees and expedite their settlement in other countries.

A Western diplomat said, however, that more ships like the Hai Hong are likely to arrive with still more refugees, especially since Vietnam apparently has no objection to the departure of ethnic Chinese and the Chinese, whose wealth grew under previous U.S.-backed governments, can afford to buy their way out.

Malaysia, is particularly eager to avoid an influx of Chinese because, aside from the burden of caring for them, more than a third of the Malaysian population of 10 million already is of Chinese origin.