Well... unless the car is tuned with a MAP sensor, and not a MAF sensor, the ECU has already calculated the fuel delivery based upon the airflow going thru the MAF. The moment the BOV opens and vents to atmosphere, a great deal of the air that was used in the calculation for fuel delivery is no longer going into the engine, creating a very rich moment before the transition from boost to vacuum has settled. VTA can produce backfires for no reason.