Lt. Col. Joseph E. Vasco Jr., second in command of the Prince George's County police force, was seriously injured early today when his car went out of control and crashed into a tree on St. Thomas Church Road near Upper Marlboro.

County authorities said Vasco was trapped in the auto for a short time after the 12:30 a.m. accident, and rescue workers had to cut away portions of the car to free him.

He then was flown by U.S. Park police helicopter to Prince George's General Hospital where he was reported in critical condition. A hospital spokesman refused to detail Vasco's injuries, but said that his vital signs were stable.

Police officials said early today that Vasco was off duty and alone in his police cruiser when the accident occurred, but could not provide no further details.

For the last six months of last year, Vasco, a veteran detective and administrator with 17 years on the county force, served as acting chief while County Executive Lawrence J. Hogan searched for a successor to Chief John W. Rhoads, who had retired.

Though Vasco had been an early candidate for the chief's job, Hogan abruptly dropped him from consideration, saying he wanted a chief that he felt would be more acceptable to the county's black residents.

Vasco's reputation had been marred by his alleged role in a controversial police operation whose members came to be widely known on the department as the "death squad."

In February, 1979, The Washington Post reported allegtions that the so-called "death squad," a group of police detectives that included Vasco, had used police informers to set up convenience store robberies in order to catch holdup men. Two suspects were fatally shot by police in the subsequent holdups.

Vasco denied the allegations, and an internal county police investigation cleared him of any wrongdoing. A four-month-long Maryland State police investigation, concluded last October, however, confirmed Vasco's role in planning the operation.

Vasco was replaced as acting cheif in late December by former FBI man and senior Hogan aide John E. McHale. McHale was named chief on Feb. 6.