Prince George's County Assistant Police Chief Joseph D. Vasco Jr. testified in federal court today that there are certain "hypothetical" occasions when it is proper for police to "provide the time, place and opportunity" for suspects to commit crimes.

But Vasco denied that such "hypothetics" apply to a series of convenience store robberies and burglaries in 1967 for which he and two other county officers are accused of illegally instructing informers to recruit participants.

Two suspects in the so-called "death squad" incidents in 1967 were shot and killed by waiting police. A third was wounded and seven others were arrested. The families of the two slain suspects, plus two of the arrested suspects, are suing police for $9 million, contending in the current civil trial here that the officers violated their civil rights and denied them due process under the 14th Amendment.

Under cross-examination by plaintiffs' attorney Charles L. Richards, Vasco said today that, "hypothetically speaking," if police knew that a certain group of suspects was committing armed robberies, "I would provide the time, place and opportunity" for them to engage in a robbery where police could stake out the targeted site and catch them.

Asked if he would send an informer into the group with instructions to solicit members to commit a crime, Vasco said, "I would have no problem with that."

But he repeatedly denied that anything like that happened in the series of "death squad" incidents. Both Vasco and the other two police defendants--Capt. James Fitzpatrick and retired Maj. Blair Lee Montgomery--have testified throughout the five-month-long trial that informers came to them with tips about planned robberies and burglaries and that detectives then routinely staked out the targeted stores.

They also claim that suspects were shot and killed or wounded only after ignoring orders to halt and firing or pointing handguns at police on the scene.

Earlier today, Vasco, saying he had misunderstood the date of a 1982 article in The Washington Post that quoted him, changed his testimony about the quote. He was originally questioned about the quote last week by plaintiffs' attorney Barnet D. Skolnik, who contended the quote suggested Vasco thought it was proper in 1967 for police to stage robberies and burglaries to catch suspects.

The quote, published last July 28, said, "You have to understand that these are the kind of things that were done. I mean, the kind of police work that was done 15 years ago in a different era, a different culture. There was nothing wrong with it then. I mean, there's nothing wrong with it now."

Vasco responded last week by saying, "At least 50 percent of what The Washington Post quoted me as saying is totally inaccurate."

Today, he said he mistakenly thought the quote had been published in a series of 1979 Post stories originally outlining the "death squad" allegations and that was why he branded the quote as "at least 50 percent. . . inaccurate."

He said today the quote was substantially accurate but that its context was unclear or wrongly suggested that he was being asked by the Post reporter to respond to the "death squad" allegations. Vasco said he had refused to discuss the allegations as a ground rule of the interview and that the quote was made after the reporter asked him how the current "death squad" lawsuit was affecting his job and family life.