For months, Mark Vasconcelles had lived in fear for his life, the victim of threats and harassment from a former high school buddy--a psychiatric patient who blamed him for his problems.

At 5:16 p.m. Friday, Vasconcelles, 29, a public information officer for Sangamon State University, was shot as he left work. He fled, trailing blood through a university parking lot in a desperate race for life. Then, after Vasconcelles had fallen, he was killed with a second shot in the back of the head, authorities said.

Described by friends as a ''quiet, gentle kind of man,'' Vasconcelles was buried Monday afternoon about the same time his former classmate was arraigned on murder charges in a Sangamon County courtroom.

Tim A. Ferguson, 30, of rural New Berlin, surrendered to a city police officer 45 minutes after the shooting. According to a police report, Ferguson walked up to the officer and said, ''I`m the one they`re looking for. I`m the one who did it.''

Associate Circuit Judge Jeanne Scott ordered Ferguson held without bond pending a preliminary hearing on four counts of murder.

Shackled and dressed in a green jail uniform, Ferguson appeared unemotional as Scott handed him copies of the four charges.

In a low voice, Ferguson told the court that he had no assets other than a 1978 Honda Accord, in which police found the weapon they said was used in the killing. Ferguson, who was assigned an attorney by the court, said he had a savings account, but ''there is nothing there; I just keep it open.''

Ferguson was diagnosed as a chronic paranoid schizophrenic in 1981, after he stabbed another high school classmate. Authorities said he apparently blamed Vasconcelles and the other classmate for the downturns in his life.

''There was no friction between them in school, none at all,'' said Alan Fox, now principal of Pleasant Plains High School. He was a junior high school teacher when Ferguson and Vasconcelles were in the 7th and 8th grades.

''I think the problem with the Ferguson boy all stemmed from Vietnam,''

Fox said. ''He just got messed up somewhere, someway. I know the difficulties began after he came back.''

In 1981, Ferguson pleaded guilty but mentally ill to aggravated battery and spent 337 days at the Chester Mental Health Center, said Tom Conway, deputy chief of investigations for the Springfield Police Department. Upon his release, Ferguson was ordered to undergo further treatment at the Danville Veterans Hospital and was put on probation for 30 months. The probation period ended March 16.

''He blamed them for many of his problems,'' said Conway, referring to Ferguson`s high school classmates. ''We had at least two reports since July from Mr. Vasconcelles that the suspect was trying to contact him and was threatening him.''

Other police and university officials said they believed Ferguson had been stalking Vasconcelles for even longer periods.

''It`s really shocking to think that the guy could have been stalking him for as long as 10 years,'' said a university employee who knew Vasconcelles well.

Durward Long, president of Sangamon State, said police and university security personnel had been notified.

''He made all the correct efforts to prevent the person from carrying out the threat,'' Long said of Vasconcelles. ''When one looks at the circumstances, one would have to conclude that no matter where it took place, it could not have been prevented short of taking the assailant out of circulation.''

University officials learned of the shooting Friday evening as they were leaving a faculty reception at the university`s public affairs center. Vasconcelles, also a graduate student taking classes in human relations and social psychology at the university, apparently was shot first in the right arm with a 20-gauge shotgun. While trying to flee, he was hit in the back of the head with another shot, said Sangamon County Coroner Norman Richter.

Detective Donald Stouffe testified that Vasconcelles had managed to run about 300 yards before he fell. At the same time, Stouffe testified Monday, Ferguson was reloading his shotgun. Then, at a range of no more than seven feet, Ferguson shot Vasconcelles in the back of the head while he lay in the parking lot, Stouffe said.

On Sunday night, about 200 friends and university colleagues gathered in downtown Springfield for a candlelight service for Vasconcelles. They sang

''Amazing Grace'' and ''Kumbaya'' in soft voices to the accompaniment of a guitarist.

Vasconcelles, a Springfield native and former employee of the Southern Illinois University School of Medicine in Springfield, also was on the board of directors of the Rape Information Crisis Service and Community Energy Systems. He was a graduate of Western Illinois University at Macomb.

A memorial service for Vasconcelles will be held at noon Sept. 5 at the university, and Long already has announced that a scholarship for

communications students will be established in his honor.

Vasconcelles had been active in the Springfield community and had volunteered for arts organizations, social service agencies and the Peace Coalition, friends said. He was in his second year as a public information officer at the university.

''You hear about these kinds of things in schools in Chicago and St. Louis,'' Fox said. ''When people you`ve taught are involved, it makes you think scary thoughts.''