A Vacuum breaker is a device (several types) that is placed above the critical level
normally ABOVE the flood level rim of a fixture Or a certain height (depending on local codes) above the highest sprinkler head in irrigation to prevent back syphon of water of a questionable quality into the potable system.
A Vacuum breaker is also used on some heating and tank applications to allow air into this vessel to prevent collapsing when these tanks cool down they can cause a vacuum severe enough to cause an implosion
For example I replaced a steel tank on a 59th floor equipment room when someone drained it in the basement with total disregard to allowing air to fill the void being created INSTANT CRUSHINGGGGGGGGGGGGGG..
If you have any more detailed questions ASK JACKASSJACK He knows everything as he told me so.