OldBoy

It doesn't matter if you're using the same vacuum port that feeds the BOV. That vacuum line only serves to counter the pressure of the spring in the BOV so you can vent excessive pressure in your charge pipes. That line is never vented to atmosphere. It represents the pressure in your intake plenum at all time.

When you're boostin' you pressurize your plenum. When you suddenly let go of the throttle, the throttles close and a vacuum is created in your plenum but pressure is still being built in all your charge pipes before the throttle. If the BOV wasn't there, there's a good chance you'd blow your couplers off from the pressure.