The son of a former South Vietnamese ambassador charged here in the murder of his parents was ordered yesterday to undergo a mental examination to determine whether he is competent to waive his right to an insanity defense.

D.C. Superior Court Chief Judge Fred B. Ugast ordered the examination of Tran Van Khiem, 61, after a pretrial hearing in which Khiem told the judge he believed there was a conspiracy to force him to use the insanity defense against his will. During the hearing the prosecution also introduced a series of letters Khiem had written to President and Mrs. Reagan and to newspapers.

In the letters, dating from 1983, Khiem said alternately that he came to this country from his Paris home at his parents' request and that of Nancy Reagan. Khiem told the president in one letter that he was at "your disposal" and that only he could prevent Reagan's inevitable assassination. In another, he linked President Kennedy's assassination to his own imprisonment in South Vietnam.

Khiem was indicted in April on two counts of first-degree murder in the July 1986 suffocation deaths of his parents, Tran Van Chuong, 88, and Nam-Tran Chuong, 75, whose bodies were found in a bedroom in the Northwest Washington home they shared with their son.

Tran Van Chuong, the South Vietnamese ambassador, and his wife, who served as South Vietnam's permanent observer to the United Nations, gained widespread attention in 1963 when they resigned their posts to protest the treatment of Buddhists by the government of South Vietnamese President Ngo Dinh Diem. The Chuongs' daughter, Madame Ngo Dinh Nhu, was Diem's sister-in-law and was widely known as the "Dragon Lady" for her rigorous role in support of the Diem regime.

Khiem waived his right to raise the insanity defense at a hearing in May, but prosecutors later asked the judge to review whether he was competent to make this decision in light of the series of letters. Khiem has denied killing his parents and attributed their deaths to falls.

Ugast said yesterday he found Khiem, a lawyer in Paris and a former paramilitary official in South Vietnam, very intelligent and articulate. But he said that although he thought Khiem was "fit to stand trial," he did not know whether he was competent to waive the insanity defense.

The judge also rejected a defense motion to dismiss the indictment and a prosecution motion to allow Dr. Lechi Oggeri of Fayetteville, N.C., one of Khiem's sisters, to recount conversations she had with her mother the night before she died, in which the mother allegedly described her fear of Khiem.