The atmospheric vacuum breaker/backflow preventers are not designed to have pressure on the downstream side of them. Your system has the valves after the VB so it always has pressure on it. The poppet floats on the water during periods of flow and drops down when the flow stops. Having continual pressure can bond the poppet to the seat, and since it is not spring loaded it probably would not drop down to open the vent in a backflow condition. The valves with vacuum breakers do comply with the requirement, as long as the valves are installed at least 12" higher than the highest sprinkler head and at least 12" above the ground. Unless you only have a few zones, installing a central pressure vacuum breaker will probably be less expensive than all new valves and the piping to accomodate them.