The Montgomery Township Supervisors have voted to buy a vacant field across Stump and Horsham Roads from the township administration building. They are buying it from Univest Bank, which took possession in a foreclosure action.  A developer had bought it, planning to build an office complex, but the plans fell through when the economy went bad.  The township is paying $1.5 million for nearly 13 acres, and in the future the property could become home to a library, community center or sports facilities, possibly including a swimming pool.