Isn't a dump valve vacuum operated? Hence the vac pip connected to it?

All thats needed is a valve to stop the vac when ALS is in use.

Having said that, ALS just keeps the turbo spinning so the DV will release when it's upper boost limit is reached, the turbo on rally car will boost enough to keep it open while the gear is changed since it's tiny amount of time. Giving almost full boost as soon as the throttle is opened again since the inlet is at full pressure already.