A vacume brake is a litle flaper valv that sits on top of a pype to let air in to stop keeping air from geting in so that water and stuf can go down the pipe insted of keeping it in the pype. U can get good cheep ones made of plastic.
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.
Vacume breaker is placed inline or on a tap. Say you have a hose, one end on a tap the other sitting in some chemicals (weed killer etc). If the mains burst, its possible the vacume will draw the chemicals back into the mains. THe vacuma breaker picks up the pressure difference change, and introduces air into the system to break the vacume/syphone