[Richard]

I just realized that I misled the solution. I did not tape the sink openings closed, that would not have helped. The vacuum breaker is after the P trap. You have to go under the sink and tape the opening of the vac breaker closed. Mine had two slots.

Supposedly they are threaded into the plumbing and therefore replaceable. I don't see any evidence that glue was applied to the threads, but they will not budge. I don't want to risk tearing the plumbing loose so I will stick with the tape for now.