There are a couple of different vacuum breakers (3 that I've used) for the shower
one is built into the mixer valve, Odds are there is a rubber flap and you can remove the flap (Take the hose off) and clean or invert it.
one is the diverter in a diverter type tub filler/shower mixer.
The third is an "inline" like the ones you screw on the faucet at the perk before you hook up your hose (If you do as you shold). This one can be removed.