A U.S. District Court judge in Alexandria ruled yesterday that the information from warrantless searches and electronic surveillance approved by President Carter and Attorney General Griffin B. Bell may be used in criminal prosecution of two men accused of spying for Vietnam.

Judge Albert V. Bryan Jr.'s ruling marks the first time that a federal court has considered the overt introduction of criminal evidence obtained without warrants during a national security investigation, according to lawyers for both defense and prosecution.

"It is certainly the first time the government has ever tried to build its case on a warrantless wiretap without even considering going for a warrant," said John Shattuck of the American Civil Liberties Union.

The government has maintained that such a move was necessary because current laws applying to the case would have compromised secrecy of the investigation if a warrant had been sought.

Although there have been related decisions in federal district courts, the Supreme Court has never ruled on the legality of such evidence when obtained during an investigation of foreign agents.

Most of the government's case against former Vietnamese antiwar activist David Truong and Ronald L. Humphrey, the U.S. Information Agency employe accused of stealing classified documents for Truong and the Socialist Republic of Vietnam, depends on the warrantless wiretapping, bugging and searches conducted during the investigation.

Another key element in the prosecution's case is the testimony of Dung Ktall - daughter of a former Vietcong ambassador to the Soviet Union - who works as a paid informant for the Central Intelligence Agency and the FBI.

The prosecution said yesterday in a letter to defense attorneys that krall was given $20,200 by the government between June 1976 and yesterday. She is still receiving $1,200 month, plus a per diem and travel and living expenses when she is on assignment.

"Before she would agree to testify," the government said in the letter, "Mrs. Krail wanted to be sure that her family was safe and sought the assistance of the governament. The government spent $11,800 in this regard."

U.S. Attorney William B. Cummings told a reporter he did not think these payments would be a problem. "You don't get the information free, generally, do you?" he asked.

Humphrey's attorney, Warren L. Miller, disagreed. "Any reasonable person would have to ask whether Mrs. Krall's testimony and veracity are going to be influenced by such substantial amounts of money."

Bryan's wiretap ruling and the Krall revelations were the latest developments in a controversial case that has already resulted in removal of Vietnam's ambassador to the United Nations and the disclosure of extensive details about U.S. counterintelligence operations. The lawyers involved have begun, literally, to shake their heads at its complexities.

Trial date is still a month away.

Earlier in the week the government said it had intercepted telephone calls from Humphrey in a previous national security investigation. Bell submitted a personal affidavit claiming that he has the "privilege" to keep the information from that surveillance a secret.

But Bryan yesterday ordered that the material be handed over to the defense because it might be exculpatory in the espionage case.

"I haven't seen the tapes yet, but I can only surmise that this may be related to the Alekseyev affair," Miller said. He was referring to his client's cooperation with the FBI during the investigation of a Soviet agent named Vladimir I. Alexseyev during the same period Humphrey was allegedly spying for the Vietnamese.

Defense lawyers for both Truong and Humphrey have regularly protested what they regard the government's excessive secrecy.

"Bryan's decision on the surveillance and searches was made on the basis of documents which we have never seen," said one of Truong's attorneys. "It was made in secret without the defendants' attorneys. The right to adversary proceedings is destroyed in such a case."

Yesterdy's ruling holds that as long as warrantless wiretaps, bugs and searches are part of a national security investigation of foreign agents - and not made with the "primary purpose" of prosecution - the evidence can be used in court.

The key determination for the defense, in that case, is exactly when the government decided to prosecute.

Bryan sets the date at July 20, 1977, and says that all the warrantless surveilance and searches after that are illegal - but most of the incriminating evidence in the case was discovered by the end of June.

According to Bryan's decision, the date was chosen on the basis of letters from Bell to CIA Director Stansfield Turner and National Security adviser Zbignlew Brzezinski requesting them to "make the necessary documents and witnesses available for use at Truong's and Humphrey's) trial."

Defense attorneys have never seen those letters.

Under normal court rules defense lawyers cannot immediately appeal Bryan's rulings. But one of Truong's lawyers said, "In the event of a conviction, we will, of course, go all the way to the Supreme Court."