Prince George's County police sergeant testified today that retired officer John R. Cicala, a key witness against Deputy Police Chief Joseph D. Vasco in the so-called "death squad" trial here, vowed to "get back" at Vasco, apparently in the belief that Vasco had blocked his medical disability retirement in 1977.

Sgt. John M. Calhoon told a federal jury he overheard Cicala tell a group of acquaintances at a party in early 1976 or 1977: "That bastard Vasco did this to me, and some day I'm going to get back at him."

Vasco and two other police defendants, accused in a $9 million civil rights lawsuit here of staging several robberies of convenience stores in 1967 in which two suspects were shot and killed, contend Calhoon's testimony undermines Cicala's credibility by showing he was personally motivated against Vasco.

Cicala testified last December in the nearly four-month-long trial that Vasco and others planned the robberies "from A to Z."

He also said he was fired, but later reinstated, for refusing to pose as a clerk in one of the convenience stores staked out by police. He said he feared for his life but also felt it was "legally and morally wrong."

Under cross-examination by police attorneys in December, Cicala acknowledged he was embittered by his dismissal in 1967 as well as the county's refusal 10 years later to grant him a work-related medical disability retirement, which would have given him more pay than an ordinary retirement.

Calhoon testified today that Cicala was apparently referring to his retirement status when he vowed to "get back" at Vasco in early 1976 or 1977, by which time Vasco was a high-ranking official of the police department and presumably could have influence on Cicala's retirement status.

Under cross-examination by the plaintiffs' attorney, R. Kenneth Mundy, Calhoon acknowledged he had never met Cicala socially or professionally but nevertheless recognized him at the party in 1976 or 1977. He said he did not tell attorneys for Vasco about the incident until last December after he read news accounts of Cicala's testimony in the present trial.

"He said he didn't want to be in court," Calhoon testified, " . . . but I knew that was wrong and that he had a motive to testify [against] Vasco."

"Why did you not come forward sooner?" asked Mundy.

"My memory was jogged by reading his testimony," said Calhoon, adding that even though he had read accounts of the so-called "death squad" incidents as far back as 1979 in The Washington Post, he did not recall the party with Cicala until last December.