A former U.S. Information Agency officer and the son of a once-prominent Vietnamese politician were sentenced yesterday to 15 years in prison on charges that they spied for Communist Vietnam.

A stern-faced U.S. District Judge Albert V. Bryan Jr. quickly listened to the men's pleas for mercy and then handed out identical sentences to Ronald L. Humphrey, 42, and David Truong, 32, saying both had committed serious crimes.

"In the name of God, I ask for mercy," said Humphrey, a career foreign service office, who had admitted passing classified State Department cables to Truong. His voice quaking with emotion, Humphrey told the judge he had been "unjustly convicted of a wrongdoing" far less serious than espionage, one of six crimes of which he and Truong were found guilty by a jury on Mary 19.

Truong, whose father had run for president of South Vietnam on a peace ticket in 1967, also pleaded with the judge for a light sentence. "I did not wrong anyone in this country - or this country." Truong said, reading from a handwritten statement.

But Bryan, who, had presided over the two men's three-week trial in Alexandria, said, "There is no need for me to make any further remarks. All that can be said has been said in this case or more than needs to be said."

He gave the men sentences totaling 47 years for breaking six separate laws, ranging from espionage to theft of government property. But then he directed that the six sentences run concurrently, meaning that the longest either man could stay in prison is 15 years.

Asuming good behavior, Humphrey, who has been in jail since his arrest Jan. 31, will be eligible for parole in four years, according to a Justice Department lawyer. Truong could face a slightly longer prison sentence because he spent several weeks free on bond before his trial.

Each man faced sentences that could have been as severe as life plus 35 years. "I think the sentences were as light as they could have hoped for," said Frank Dunham, an assistant U.S. attorneys who handled part of the prosecution.

Defense lawyers, who had earlier announced they would appeal the convictions, expressed disappointment at the sentences. "We're not surprised at the sentence, given the magnitude of the crime, but we're disappointed that the court did not see fit to give him (Humphrey) a more lenient sentence," said his lawyer, Warren L. Miller.

The judge agreed there were "mitigating factors" favoring Himphrey, who said he gave the cables to Truong in hope that he would speed the release of his common-law Vietnamese "wife" from a Communist prison.

Truong, who passed the cables to a Vietnamese woman who was a double agent for the CIA, was "in my view the more culpable of the two," the judge said. Bryant said he would have been more severe with Truong if he had been an American citizen. But, he added, "He has abused the hospitality of the United States."

The sentencing, like the trial of the two men, was not without its moments of drama. The small Alexandria courtroom was packed with newsmen and spectators long before Bryan and the defendants entered the chambers.

Among them was the slender Vietnamese woman Humphrey called "Kim" and credited with saving his life while he was stationed in Vietnam during the war there. Neither she or Humphrey showed any emotion as Bryant imposed the sentence, but moments later there were tears streaming down her ashen cheeks as she was led from the courtroom.

Humphrey, a $30,000-a-year evaluations officer at USIA headquarters in Washington when he was arrested repeated to the judge yesterday his claim of innocence. "I cannot feel remorse for crimes not committed. I am not a spy, not a conspirator, not a foreign agent," he said in a statement that he read to the judge and had mailed to newsmen prior to the sentencing.

Both men had contended they were victims of a political trial, prompted by an administration anxious to prove it could deal sternly with Communist countries and ready to use extensive wiretapping and electronic surveillance to prevent spying. President Carter and Attorney General Griffin Bell personally approved the use of secret television surveillance of Humphrey at his USIA office and of planting listening devices in Truong's Washington apartment.

Although those steps provided the grounds for much of the pretrial maneuvering, the electronic data-gathering never was an issue in the actual trial.The defense lawyers have said they will question the extent of the surveillance again when the case is heard by the 4th U.S. Circuit Court of Appeals.

While both men admitted their role in transmitting the documents they testified during the 11-day trial that their motives were not criminal. Humphrey said he did it out of his love for Kim and Truong said he hopes his actions would improve relations between the U.S. and Vietnam.

The documents they were accused of sending through the double agent to Vietnamese officials in New York and Paris were of little importance, both men had insisted throughout their trial. Yesterday they renewed that arguments before Bryan.

"This was not a spy case with secret decoder rings . . . everything was out in the open," said lawyer Marvin Miller, who represented Truong.

"Your honor," said Warren Miller, representing Humphrey, "There is espionage - and there is espionage. In this case the (State Department) documents were low-grade . . . The motivation of Mr. Humphrey was not for profit, but for the love of a woman and her children."

Humphrey, his lawyer said, is "the type of person described by others as naive, overly trusting, but always eager to help others." Humphrey had claimed he was unaware that Truong was funneling the documents to Vietnamese officials, but admitted he erred it taking the documents from USIA headquarters.

"I feel remorse for my bad judgment and the misery it has caused." Humphrey said at one point yesterday.

"My mind is totally at peace and I'm ready to climb the highest mountain and go down to the lowest peaks no matter how long it takes," Truong told the judge.