To his friends in Washington, Ronald Louis Humphrey was one of the volunteer workers in a newly formed foreign correspondents association whose efforts were joined by those of his wife, an assistant for a German television bureau in Washington.

To his neighbors in Arlington, however, Humphrey was the recently returned career diplomat with a Vietnamese "wife" and four children.

Yesterday, as Humphrey stood in a federal court accused of spying for communist Vietnam, the contrasting aspects of his private life provided yet another puzzling aspect to an espionage case one lawyer denounced as "ballyhoo" by the FBI.

In court along with a Vietnamese national accused with him of passing classified documents to Hanoi, Humphrey portrayed himself as a financially hard-pressed, but loyal American diplomat who was attempting to support a "common law" Vietnamese wife and her children.

His pleas for a reduction in his $250,000 bond won some support from District Court Judge Albert V. Bryan Jr. who ordered his bond cut to $150,000. But Bryan was unmoved by defense charges that the case against Humphrey and David Truong, also known as Truong Dinh Hung, was built on theft of documents that "are the kind you read about in Jack Anderson, James Reston, The Washington Post or The New York Times."

The judge refused to reduce the bond for the 32-year-old Truong, the son of a Saigon lawyer who was once a major political figure in the country before the Communist takeover in 1975. The charges represent "serious, extremely serious offenses," Bryan said.

Neither Marylou Humphrey, who said she was separated from her husband in 1969, nor "Kim Humphrey," his gaunt Vietnamese "common law wife," were in the Alexandria courtroom as both Humphrey and Truong pleaded innocent to charges of espionage, conspiracy and other offenses.

In the Arlington Heights neighborhood where Humphrey lived with the Vietnamese woman, neighbors said they were as much shocked by the charges as by the revelation that Humphrey was not married to the woman they had just met at a holiday party as his "wife."

"I had no idea that they weren't married," said one resident who asked not to be identified.

In Washington, Humphrey's fellow workers at the U.S. Information Agency's headquarters expressed similar surprise. "I'm reading more in the paper about Humphrey than I knew about him," said one supervisor.

"Kim Humphrey" and Humphrey's mother, Leah Humphrey, defended Humphrey in brief interviews. "I didn't know what happened," said Kim Humphrey. His mother pointed to a photograph of her son with the late U.S. Sen. Hubert H. Humphrey at a reception in Germany. "There is no way that he could have done this awful thing," he said.

The photograph was autographed by the senator and his wife and was hunging in a corner of the modestly furnished living room of the Humphreys' rented home at 618 S. Irving St.

Although State Department officials continued to speak of the incident in harsh terms, a spokesman said the department does not expect the arrests to block future meetings between the U.S. and Vietnamese governments. At the conclusion in December of the last round of negotiations between the two one-time foes, both governments had agreed to continue meetings at a later date.

The Paris meetings had generated plans for a Vietnamese delegation to visit Hawaii this spring to study U.S. methods for identifying the remains of American servicemen believed buried and unaccounted for in Vietnam.

The State department continued yesterday to press its formal complaint over "the involvement" of Vietnamese officials as signed to the United Nations "in espionage directed against the United States." The protests were seen as a prelude to possible expulsion of Dinh Ba Thi, the chief of the Vietnamese mission from New York, who was named as an unindicted co-conspirator in the case.