There are several types of back flow prevention devices. A vacuum breaker as you have pictured is one of these. The valves that need to be on are the ball valves on each side of the device. The bleeders are part of the test cocks. When the device is tested, the inspector connects his instrument to the test cock and opens the bleeder. If you turn water on with a bleed open, it will squirt water. Usually they open/close with a flat screwdriver. No harm done if water applied while open. I have not used a vacuum devices as you have shown, but I'm sure the water pressure will activate the device. Some cities require annual testing of the back flow devices by a licensed inspector, so you should check codes in your area.