shellip,
A vacuum breaker is a system that allows air in the plumbing and negates any possibility of the flow of water from the pool back thru the plumbing to the pit. Because the pool elevation of water is higher than the pit, water seeks to level itself, thus draining water out of the pool until the water levels are equal.
Vacuum breakers are commonly used on hose bibs and sprinkler systems that are attached to any potable water supply line. It's part of the building code here in CA.

Check valves serve a similar function for pools but occassionally can be defeated by debris being caught inside the valve. It's a good practice to use redundancy and put check valves on the suction side and the return side after the filter, if you are using a filter on this pump. It's harder to defeat the valve if you are running filtered water thru it.

PS. I think you will find it more cost effective to use a couple plastic check valves than a single 2 or 2-1/2" brass vacuum breaker.