The Fairfax County Board of Supervisors yesterday endorsed the idea of citizen patrols in residential neighborhoods after being told that such groups help reduce vandalism and burglaries of homes.

The county police department told the board that in the North Springfield community, where a citizen security patrol was started in early 1976, the number of reported burglaries dropped from 71 to 42 during comparable 10-month periods. The number of reported cases of vandalism in the same neighborhood decreased from 94 to 79, police reported.

Nine communities in Fairfax already have organized security patrols to combat burglaries and larcenies, which last year accounted for 89 per cent of all serious crimes in the county, according to country board chairman John F. Herrity.

Supervisor Martha V. Pennino (D-Centreville), calling the citizens' patrol "an idea whose time has come," noted that when she first proposed the same idea two years ago "it was ridiculed and called vigilantism."

In order to avoid the charge of vigilantism the supervisors yesterday also adopted guidelines for the patrols' activities, based on those drawn up by the North Springfield patrol.

Prodded by the rising number of burglaries in their neighborhood, residents of North Springfield decided to form a patrol in November, 1975, according to Peter T. Via, of Long Pine Drive, one of the patrol's founders. Most of the community's burglaries were committed by juviniles.

Via said that the petrol was organized in cooperation with the county police who "were very sympathetic." He said that "we have the greatest respect for Col. Richard A. King [the Fairfax police chief] and his police force, but they aren't ubiquitous."

Via said about 80 volunteer residents of North Springfield, both men and women, are called upon twice a month to participate in the patrol. Two cars equipped with CB radios drive the streets of the 2,000-family community for four to six hours a day. If they observe any suspicious acitvity they relay their observation to a base operator via CB radios. The base operator then places a call to the nearest police station.

The operating rules of the patrol forbid the patrollers to leave their cars, to carry weapons or to drink alcoholic beverages while on duty, Via said. They are not allowed to give chase or arrest suspects and "their only function is to observe and report [any suspicious activity]," Via said. Would-be burglars are warned by signs stating that North Springfield is a "patrolled community."

Volunteers for patrol duty carry notarized membership cards after passing tests at the end of a six-hour training session. They are taught safety practices and how to operate CB radio equipment, Via said.

Even as they endorsed the citizen participation in patrolling communities, several supervisors urged that the county put a higher priority on police and fire protection.

"I would like to see the board and staff view the citizen patrols as a challenge to see what we can provide in the way of increased services and patrols," said supervisor Joseph Alexander (D-Lee).

In a county as wealthy as Fairfax, the government "ought to spend a little bit more money so citizens . . . can feel safe in this county," Alexander said. Supervisor James M. Scott (D-Providence) said he thought the neighborhood patrol, by involving neighbors with neighbors, have the bonus of building a sense of community "which is one of the problems in this sprawling country of ours."

Herrity said that in addition to the North Springfield patrol, county residents are organized into patrols in the Villa May, Belle Haven, Pinewood Lake, Pinewood Green, Friendly Village, Sequoia Village, West Springfield and the Greenbriar/Brookfield-Brookside neighborhoods.