The Arlington County Board announced last night that it has arrived at an agreement expected to end the suit filed by former County Manager W. Vernon Ford over retirement benefits due him after he was fired 21 months ago by the then Republican-controlled board.

The board, now controlled by Democrats, voted 3 to 2 to give Ford, 52, full retirement benefits. The county's retirement board must approve the proposed out-of-court settlement.

"We have granted him the full retirement benefits permitted by law," said Board Chairman Ellen M. Bozman. Democratic board member John G. Milliken added that Ford's benefits will require "no out-of-pocket money" from the county because early retirements are a built-in assumption of the pension system.

"It's what I asked for in the beginning," said Ford, who added that he would drop his suit if the agreement is approved. "It means I will have been treated as other county employes have been treated."

Ford was fired in August 1981 by the three Republican-backed independents then on the board. The three had clashed with him frequently over implementation of their policies, particularly on economic and development issues. Bozman and Milliken, the Democrats on the board then, charged that Ford's firing was "backroom politics."

Democrat Mary Margaret Whipple complained about what she called the lack of "open government" in her successful campaign last year against then-Board Chairman Stephen H. Detwiler.

Board members Dorothy T. Grotos and Walter L. Frankland, Republican-backed independents, voted to dismiss Ford, and voted against giving Ford full benefits. Ford served 22 years in county government before being named county manager in 1976.