The town manager of Vienna for the past seven years, Brackenridge (Brack) Bentley, announced his resignation yesterday after a year of turmoil in the Fairfax County town of 18,000.

Bentley, 53, said he was taking "a more challenging job" Feb. 1 as manager of a larger Virginia municipality, at roughly his current annual salary of $47,820. Bentley said he could not divulge the name of the locality until after Dec. 10, under agreement with his new employer. He will leave his Vienna post on Jan. 25.

Most town and city managers last an average of four years before moving on, said Bentley, "and I think it's time for me to go."

He said he has enjoyed a friendly and trusting relationship with the Town Council "and I want to leave while that trust is still there." He said he had not been asked to leave, despite a recent citizen petition drive calling for his resignation.

Bentley said that because of the firing of two police chiefs in less than a year and widespread dissension within the town's 31-member police force, "I think there is a group out to get me" and members of the Town Council. "My departure should take some wind out of their sails," he said.

C. Eugene Clarke, chairman of the Voters for a Better Council, which has criticized Bentley's management and the council's leadership, said yesterday, "We're pleased with his decision. It's best for him and the town. But Bentley is the least of the problems of Vienna."

Clarke said major issues facing the town are the "overwhelming traffic problems coming with Tysons II and Tysons III and the Metro subway station . . . which this reactionary council" has failed to anticipate.

Vienna Mayor Charles A. Robinson Jr. declined to comment on Clarke's charges, but praised Bentley as "a very professional manager" who presided in "a time of turmoil in the police department and adverse publicity for the town . . . which perhaps convinced him he could make a better contribution in another city."

Robinson said that except for pending lawsuits by fired police officers, the dissension in the police department has ended with the recent appointment of former Fairfax County police major Donald G. Harper as chief. Robinson said, "Harper has won widespread respect and improved morale in the department."

He said Vienna will immediately begin a search for a manager and probably will have to appoint an interim manager because the search will take several months.